Science Class Ten

Chemical Reactions and Equations

Types of Chemical Reaction

Chemical reactions can be classified in following types:-

(a) Combination Reactions

(b) Decomposition Reactions

(c) Displacement Reactions

(d) Double Displacement Reactions

(e) Oxidation and Reduction Reactions

(a) Combination Reaction: Reactions in which two or more reactants combine to form one product are called COMBINATION REACTION.

A general combination reaction can be represented by the chemical equation given here.

Example:-

When magnesium is burnt in air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.

When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.

When hydrogen reacts with chlorine, hydrogen chloride is formed.

When calcium oxide reacts with water, calcium hydroxide is formed

When carbon monoxide reacts with oxygen, carbon dioxide is formed.

(b) Decomposition Reaction: Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction.

A general decomposition reaction can be represented as follows:-

Example:

When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.

When ferric hydroxide is heated, it decomposes into ferric oxide and water.

When lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide and oxygen.

In the above examples, compound is decomposed because of heating, so, these reactions are called THERMAL DECOMPOSITION REACTION.

Electrolytic Decomposition:

Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as ELECTROLYTIC DECOMPOSITION. This is also known as ELECTROLYSIS.

Example:

When electricity is passed in water, it decomposes into hydrogen and oxygen.

Photolysis or Photo Decomposition Reaction:

Reactions in which a compound decomposes because of sunlight are known as PHOTOLYSIS or PHOTO DECOMPOSITION REACTION.

Example:

When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.

Similarly, when silver bromide is put under sunlight, it decomposes into silver metal and bromine gas.

Photographic paper has coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal.

(c) Displacement Reaction:

Reactions in which atoms or ions move from one compound to other to form new compound are known as DISPLACEMENT REACTION. Displacement reaction is also known as Substitution Reaction or Single displacement /Replacement Reaction.

A general displacement reaction can be represented using chemical equation as follows:

Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not be taken place.

Example:

When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.

When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.

When silver metal is dipped in copper nitrate, no reaction takes place because silver metal is less reactive than copper.

(d) Double Displacement Reaction:

Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

A general double displacement reaction can be represented using following general chemical equation.

Example:

When solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.

When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.

Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.

Exothermic and Endothermic Reaction

Reactions which produce energy are called exothermic reaction. On the other hand, reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic. Most of the decomposition reactions are exothermic. Respiration is a decomposition reaction in which energy is released. When quick lime (calcium carbonate) is added to water, it decomposes and releases energy. Cooking involves chemical reactions which are endothermic as cooking is possible because of heating.

(e) Oxidation and Reduction Reaction:

Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation.

Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized.

Oxidizing agent: Compounds which can add oxygen or a non-metallic compound or remove hydrogen or metallic element are known as oxidizing agents.

Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced.

Reducing agent: Compounds or elements which can cause reduction are called reducing agents.

In a chemical reaction oxidation and reduction both take place simultaneously and such reactions are also known as REDOX REACTIONS. In the word REDOX, ‘Red’ stands for reduction and ‘Ox’ stands for oxidation.

Example:

When iron reacts with air, it forms iron oxide (rust)

In this reaction, oxygen is added to iron, thus, iron is oxidized. Here oxygen is oxidizing agent.

When cupric oxide reacts with hydrogen, it gives copper and water.

In this reaction, oxygen is removed from copper and oxygen is added to hydrogen. So, cupric oxide is reduced to copper and hydrogen is oxidized to water. Cupric oxide is oxidizing agent and hydrogen is reducing agent.

When sodium hydroxide reacts with hydrochloric acid, it gives sodium chloride and water.

In this reaction, sodium hydroxide is reduced to sodium chloride since hydrogen is removed from sodium hydroxide. Hydrochloric acid is oxidized to water, since oxygen is added to hydrogen chloride and non-metallic element chloride is removed. Sodium hydroxide is oxidising agent and hydrochloric acid is reducing agent.

In this reaction oxidation and reduction both takes place simultaneously, thus it is an example of redox reaction.

Significance of Oxidation Reduction in Everyday Life:

1. Respiration is oxidation reaction in which food is oxidized to produce energy.

2. Iron gets oxidized to form rust; which leads to corrosion of iron in the long run.

3. Most of the metals react with atmospheric oxygen and it leads to formation of a layer on the metal article. The metal gets corroded in the long run.

4. Rusting of iron can be prevented by painting the iron article. This can also be prevented by applying a layer of zinc over iron article. This process is known as galvanization.

5. Fried food gets oxidized when exposed to air. This spoils the taste of the food and the food becomes unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity.

6. We are able to utilize various types of fuel because of oxidation. Oxidation of fuel helps in producing energy.

en Science NCERT CBSE

Chemical Reactions and Equations

Chemical Reaction:- The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: rusting of iron, setting of milk into curd, digestion of food, respiration, etc.

In chemical reaction new substance is formed which is completely different in properties from the original substance, so in a chemical reaction chemical change takes place.

Following are the signs of chemical reaction:

(a) Change of state of substance.

(b) Change of colour of substance.

(c) Evolution of heat.

(d) Absorption of heat.

(e) Evolution of gas.

(f) Evolution of light.

(g) Evolution of sound, etc.

Reactant and Product:

Reactant: Substances which take part in a chemical reaction are called reactants.

Product:- New substance formed after a chemical reaction is called product.

Chemical Equation: Representation of chemical reaction using symbols of substances is called chemical equation.

Example:

In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.

When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation.

In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations.

Chemical Equation is a way to represent the chemical reaction in concise and informative way.

Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.

Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both sides.

Example:

In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation.

According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.

Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.

Example:

In this example number atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.

Balancing a chemical equation:

To balance the given or any chemical equation, follow these steps:

(1) Write the number of atoms of elements present in reactants and in products in a table; as shown here.

(2) Balance the atom which is the maximum in number; on either side of chemical equation.

In this equation, the number of oxygen atom is the maximum on the RHS.

To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number of oxygen atoms becomes equal on both sides.

(3) Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.

(4) After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.

(5) Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation.

After balancing, the above equation can be written as follows.

Question: 1. Balance the given equations.

Writing the symbols of Physical States of substances in Chemical equation:

By writing the physical states of substances a chemical equation becomes more informative.

Gaseous state is represented by symbol ‘g’

Liquid state is represented by symbol ‘l’

Solid state is written by symbol ‘s’

Aqueous solution is written by symbol ‘aq’

Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.

Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.

Example:

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Chemical Reactions and Equations NCERT Text Book Questions and Answers

In Text Questions And Answer (NCERT Book) NCERT Book Page Number - 6 Question :1 - Why should a magnesium ribbon be cleaned before burning in air? Answer: Magnesium forms magnesium oxide when reacts with oxygen present in air. On the magnesium ribbon a stable layer of magnesium oxide is formed when it is stored, which prevents the further reaction of magnesium ribbon. Hence, to clean up the layer of magnesium oxide, magnesium ribbon is cleaned before burning. Question2:- Write the balanced equation for the following chemical reactions. (i) Hydrogen + Chlorine → Hydrogen chloride (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride (iii) Sodium + Water → Sodium hydroxide + Hydrogen Answer: Q -3- Write a balanced chemical equation with state symbols for the following reactions. (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride. Answer: (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water. Answer: NCERT Book Page Number - 10 Q -1- A solution of a substance ‘X’ is used for white washing. (i) Name the substance ‘X’ and write its formula. Answer – The chemical name of substance, which is used for white washing is Calcium oxide. Its common name is lime or quick lime. The chemical formula of Calcium Oxide is CaO. (ii) Write the reaction of the substance ‘X’ named in (i) above with water. When calcium oxide reacts with water it forms calcium hydroxide. Calcium hydroxide is also known as slaked lime. 2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas. Answer – (Hydrolysis of water has been performed in the activity 1.7 in the text book.) When electric current is passed through water it decomposes into its constituent components, i.e. hydrogen and oxygen gases. During electrolysis of water hydrogen gas collected in one test tube and has double amount than oxygen collected in another test tube. This is because a water molecule is formed by the combination of hydrogen and oxygen in the ratio of 2:1. NCERT Book – Page Number – 13 Q -1- Why does the colour of copper sulphate solution change when an iron nail is dipped in it? Answer:- The colour of copper sulphate solution is blue. When an iron nail is dipped in the solution of copper sulphate, it forms ferrous sulphate solution and copper metal by the displacement of copper from the solution. The colour of ferrous sulphate is green. That’s why the solution becomes green. Q-2- Give an example of a double displacement reaction other than the one given in Activity 1.10. Answer: When lead nitrate reacts with potassium iodide, potassium nitrate and lead iodide is formed. In this reaction the both the reactants exchanges their irons after reaction. This type of reaction is known as double displacement reaction. Q-3- Identify the substances that are oxidised and the substances that are reduced in the following reactions. Answer: In this reaction sodium is combined with oxygen, i.e. gains oxygen and hence oxidized. Consequently oxygen is reduced. Answer: In this reaction Hydrogen gains oxygen and forms water, hence it is oxidised. On the other hand copper loses oxygen and hence, reduced. Chemical Reactions And Equations NCERT Book Questions And Answers Exercise (NCERT Text Book)Part - II Question: 11: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions. Answer : In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions. 12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity. Answer:- In this reaction energy is supplied in the form of heat. In this reaction energy is supplied in the form of light,i.e.sunlight In this reaction energy is supplied in the form of electricity. Question -13:- What is the difference between displacement and double displacement reactions? Write equations for these reactions. Answer – When a more reactive element displaces a less reactive element from its salt solution, then it is called displacement reaction. While there is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction. In displacement reaction one displacement takes place. While in double displacement reaction two displacements takes place. Displacement reaction Double displacement reaction Question -14- In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved. Answer:- Q -15- What do you mean by a precipitation reaction? Explain by giving examples. Answer – Reactions in which precipitate is formed are called precipitation reactions. Generally in a precipitation reaction insoluble salts are settled down as precipitate. In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride. Q -16- Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction Answer: Oxidation - When a substance gains oxygen or loses hydrogen during a reaction, it is called that the substance is oxidized. This phenomenon is called oxidation. In this reaction copper gains oxygen and hence is oxidized. In the above reaction hydrogen gains oxygen, and hence oxidized. Reduction - If a substance loses oxygen or gains hydrogen during a reaction, it is that the substance is reduced. This phenomenon is called reduction. In this reaction zinc loses oxygen, and hence called is reduced. In this reaction Mangnese dioxide loses oxygen and hence is called readuced. Q -17- A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed. Answer: Copper is a brown coloured shiny element, when heated in air, it becomes black in colour. Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide. Q -18- Why do we apply paint on iron articles? Answer: When articles made of iron come in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, to prevent an iron article to come in contact with moisture present in air paint is applied. Paint prevents the iron to get rusted. Q -19- Oil and fat containing food items are flushed with nitrogen. Why? Answer: When fats and oils are oxidised, they become rancid and their smell and taste change. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid. Q - 20 -  Explain the following terms with one example each. (a) Corrosion (b) Rancidity Answer: (a) Corrosion - Metals react with oxygen which is present in the atmospheric moisture. This leads to the formation of metal oxides. In due course of time, the metal keeps on changing into its oxide and finally the whole metal is lost due to oxidation. This process is called corrosion. Example: Iron articles; like iron gates or bridges tend to rust because of oxidation by atmospheric moisture. We know that rust is nothing but iron oxide. Conversion of iron into rust leads to corrosion of the iron articles. Due to this, the iron articles weaken and finally wither away.  (b) Rancidity - When fats and oils are oxidised, their smell and taste change. This process is called rancidity. Oily food often become rancid and start giving out obnoxious smell. The taste also becomes bad. Such oily food is not fit for eating. Example: When packets of potato chips or other oily snacks are kept open for a long time; their taste and smell become bad. The oily food is no longer safe to eat.  Chemical Reactions And Equations Exemplar Problem (NCERT)And Solution Multiple Choice Questions (MCQs) Question:1- Which of the following is not a physical change? (a) Boiling of water to give water vapour (b) Melting of ice to give water (c) Dissolution of salt in water (d) Combustion of Liquefied Petroleum Gas (LPG) Answer:- (d) Combustion of Liquefied Petroleum Gas (LPG) Question:2:- The following reaction is an example of a (i) displacement reaction: (ii) combination reaction. (iii) redox reaction. (iv) neutralisation reaction (a) (i) and (iv) (b) (ii) and (iii) (c) (i) and (iii) (d) (iii) and (iv) Answer: (d) (iii) and (iv) 3. Which of the following statements about the given reaction are correct? (i) Iron metal is getting oxidised (ii) Water is getting reduced (iii) Water is acting as reducing agent (iv) Water is acting as oxidising agent (a) (i), (ii) and (iii) (b) (iii) and (iv) (c) (i), (ii) and (iv) (d) (ii) and (iv) Answer: (c) (i), (ii) and (iv) 4. Which of the following are exothermic processes? (i) Reaction of water with quick lime (ii) Dilution of an acid (iii) Evaporation of water (iv) Sublimation of camphor (crystals) (a) (i) and (ii) (b) (ii) and (iii) (c) (i) and (iv) (d) (iii) and (iv) Answer: (a) (i) and (ii) 5. Three beakers labelled as A, B and C each containing 25 mL of water were taken. A small amount of NaOH, anhydrous CuSO4 and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is (are) correct? (i) In beakers A and B, exothermic process has occurred. (ii) In beakers A and B, endothermic process has occurred. (iii) In beaker C exothermic process has occurred. (iv) In beaker C endothermic process has occurred. (a) (i) only (b) (ii) only (c) (i) and (iv) (d) (ii) and (iii) Answer: (c) (i) and (iv) 6. A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation? (a) KMnO4 is an oxidising agent, it oxidises FeSO4 (b) FeSO4 acts as an oxidising agent and oxidises KMnO4 (c) The colour disappears due to dilution; no reaction is involved (d) KMnO4 is an unstable compound and decomposes in presence of FeSO4 to a colourless compound. Answer: (d) KMnO4 is an unstable compound and decomposes in presence of FeSO4 to a colourless compound. 7. Which among the following is (are) double displacement reaction(s)? Answer (b) (ii) Only 8. Which among the following statement(s) is (are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to (i) the formation of silver by decomposition of silver chloride (ii) sublimation of silver chloride (iii) decomposition of chlorine gas from silver chloride (iv) oxidation of silver chloride (a) (i) only (b) (i) and (iii) (c) (ii) and (iii) (d) (iv) only Answer: (b) (i) and (iii) 9. Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This process is called slaking of lime. Calcium hydroxide dissolves in water to form its solution called lime water. Which among the following is (are) true about slaking of lime and the solution formed? (i) It is an endothermic reaction (ii) It is an exothermic reaction (iii) The pH of the resulting solution will be more than seven (iv) The pH of the resulting solution will be less than seven (a) (i) and (ii) (b) (ii) and (iii) (c) (i) and (iv) (d) (iii) and (iv) Answer: (b) (ii) and (iii) 10. Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved? (i) Displacement reaction (ii) Precipitation reaction (iii) Combination reaction (iv) Double displacement reaction (a) (i) only (b) (ii) only (c) (iv) only (d) (ii) and (iv) Answer: (d) (ii) and (iv) 11. Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is (a) 1:1 (b) 2:1 (c) 4:1 (d) 1:2 Answer: (b) 2:1 12. Which of the following is (are) an endothermic process(es)? (i) Dilution of sulphuric acid (ii) Sublimation of dry ice (iii) Condensation of water vapours (iv) Evaporation of water (a) (i) and (iii) (b) (ii) only (c) (iii) only (d) (ii) and (iv) Answer: (d) (ii) and (iv) 13. In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be used in place of lead nitrate? (a) Lead sulphate (insoluble) (b) Lead acetate (c) Ammonium nitrate (d) Potassium sulphate Answer: (b) Lead acetate 14. Which of the following gases can be used for storage of fresh sample of an oil for a long time? (a) Carbon dioxide or oxygen (b) Nitrogen or oxygen (c) Carbon dioxide or helium (d) Helium or nitrogen Answer: (d) Helium or nitrogen 15. The following reaction is used for the preparation of oxygen gas in the laboratory Which of the following statement(s) is (are) correct about the reaction? (a) It is a decomposition reaction and endothermic in nature (b) It is a combination reaction (c) It is a decomposition reaction and accompanied by release of heat (d) It is a photochemical decomposition reaction and exothermic in nature Answer: (a) It is a decomposition reaction and endothermic in nature 16. Which one of the following processes involve chemical reactions? (a) Storing of oxygen gas under pressure in a gas cylinder (b) Liquefaction of air (c) Keeping petrol in a china dish in the open (d) Heating copper wire in presence of air at high temperature Answer: (d) Heating copper wire in presence of air at high temperature. 17. In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature? 18. Which of the following are combination reactions? (a) (i) and (iii) (b) (iii) and (iv) (c) (ii) and (iv) (d) (ii) and (iii) Answer: (d) (ii) and (iii) Chemical Reactions And Equations Exemplar Problem (NCERT)And Solution Short Answer Type Question:19: Write the balanced chemical equations for the following reactions and identify the type of reaction in each case. (a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas. (b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water. (c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H2SO4 . (d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light. Answer: Question:20 - Write the balanced chemical equations for the following reactions and identify the type of reaction in each case. (a) Thermit reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide. (b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride. (c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine. (d) Ethanol is burnt in air to form carbon dioxide, water and releases heat. Answer:- 21. Complete the missing components/variables given as x and y in the following reactions 22. Which among the following changes are exothermic or endothermic in nature? (a) Decomposition of ferrous sulphate (b) Dilution of sulphuric acid (c) Dissolution of sodium hydroxide in water (d) Dissolution of ammonium chloride in water Answer:- (a) Endothermic. (b) Exothermic. (c) Exothermic (d) Endothermic 23. Identify the reducing agent in the following reactions 24. Identify the oxidising agent (oxidant) in the following reactions 25. Write the balanced chemical equations for the following reactions (a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogencarbonate. (b) Sodium hydrogencarbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide. (c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (Cu2 I2), liberates iodine gas and also forms potassium sulphate. 26. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction? Answer: This is a Precipitation, Double Displacement Reaction. 27. Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction. Answer: Sulphur dioxide evolves in this smells like burning of sulphur. This reaction is a Decomposition Reaction. 28. Why do fire flies glow at night? Answer: There is a luminous bag found at the lower abdomen of fire flies. The light emits from this because of the secretion of luciferase enzyme. This enzyme when reacts with luciferin light is emitted because of the presence of magnesium and oxygen due to oxidation of magnesium. 29. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change? Answer: Because of the action of a fungi, called yeast over the sugar fermentation takes place. Ethanol and carbon dioxide formed because of fermentation of sugar. Sugar found in grapes ferment because of the action of fungi, yeast when it is plucked from plant, but while grapes are on plants the action of fungi, is nullified by the self defense mechanism of plants and grapes. Thus, grapes do not ferment in the case when hanging on plant. 30. Which among the following are physical or chemical changes? (a) Evaporation of petrol (b) Burning of Liquefied Petroleum Gas (LPG) (c) Heating of an iron rod to red hot. (d) Curdling of milk (e) Sublimation of solid ammonium chloride Answer: (a) Physical change (b) Chemical change (c) Physical change (d) Chemical change (e) Physical change 31. During the reaction of some metals with dilute hydrochloric acid, following observations were made. (a) Silver metal does not show any change (b) The temperature of the reaction mixture rises when aluminium (Al) is added. (c) The reaction of sodium metal is found to be highly explosive (d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid. Explain these observations giving suitable reasons. Answer: (a) No change takes place because silver metal categorized as noble metal and does not react with hydrochloric acid in normal situations. (b) The reaction between hydrochloric acid and aluminium is exothermic, thus the temperature of the reaction mixture rises when aluminium is added. (c) Since, sodium is a highly reactive metal, thus it reacts with hydrochloric acid vigorously and produces large amount of heat. Thus the reaction between hydrochloric acid and sodium is highly explosive. Even sodium reacts with water vigrouly. (d) Bubbles of hydrogen gas are formed when lead reacts with dilute hydrochloric acid, which is seen in the course of reaction. 32. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved. Answer: Element ‘X’ is calcium. When calcium oxide reacts with water it forms calcium hydroxide. Since calcium hydroxide is a base, thus it turns red litmus blue. Reaction involve in the reaction of calcium oxide with water can be written as follows: Calcium belongs to group 2nd in periodic table. It is used in cement industry and also found in bones. 33. Write a balanced chemical equation for each of the following reactions and also classify them. (a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution. (b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas. (c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas. (d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water. Answer: 34. Why do we store silver chloride in dark coloured bottles? Answer: Silver chloride shows photo-decomposition. This means when silver chloride is exposed to light it decomposes to silver metal and chlorine gas. That’s why to prevent photodecomposition silver chloride is stored in a dark coloured bottle. 35. Balance the following chemical equations and identify the type of chemical reaction. 36. A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y. (a) Write the chemical formulae of X and Y. (b) Write a balanced chemical equation, when X is dissolved in water. Answer: (a) The chemical formula of X is MgO (magnesium oxide). The chemical formula of ‘Y’ is Mg3N2 (magnesium nitride). (b) Here ‘X’ is the magnesium oxide. Thus it gives magnesium hydroxide when dissolved in water. The chemical reaction involve in it is as follows: 37. Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why? Answer: Reaction depends upon the reactivity of metals. Copper is less reactive than zinc. Copper is also known as noble metal along with gold, silver, etc. that are very less reactive under normal conditions. This is the cause that copper does not react with dilute hydrochloric acid and consequently does not liberate hydrogen gas while zinc does. 38. A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining. (a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved. (b) Name the black substance formed and gives its chemical formula. Answer: (a) A layer of black substance, called silver oxide, deposited over silver because of exposure of oxygen present in air. Finally it forms silver sulphide with the reaction of oxygen and hydrogen sulphide gas present in atmosphere, because of which silver metal turns black. This phenomenon is called tarnish or tarnishing of silver. Reaction involve in this is given here: The silver sulphide so formed can be cleaned using toothpaste. Toothpaste having many abrasive substance. Aluminium hydroxide - Al(OH)3 is one of them. When toothpaste is rubbed over tarnished silver metal, the silver sulphide reacts with aluminium and aluminium sulphide is formed leaving silver metal shiny. The cleansing speed ups with use of hot water and sodium hydrogen carbonate. The reaction involve in cleansing is as follows: (b) The black substance formed over the silver metal is silver sulphide. The chemical formula of silver sulphide is Ag2S. Chemical Reactions And Equations Exemplar Problem (NCERT)And Solution Long Answer Type Question: 39: On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed (a) Write a balanced chemical equation of the reaction. (b) Identify the brown gas X evolved. (c) Identify the type of reaction. (d) What could be the pH range of aqueous solution of the gas X? Answer:- The brown gas so formed in this reaction is nitrogen dioxide. (b) Thus, the brown gas ‘X’ is nitrogen dioxide. (c) This is a Decomposition reaction (d) Nitrogen dioxide forms nitric acid when reacts with water. The pH value of an acid is less than 7 to 0. Thus the pH range of the aqueous solution of gas ‘X’ i.e. Nitrogen dioxide many be from 0 to less than 7. The reaction between nitrogen dioxide and water can be written as follows:- Question:40. Give the characteristic tests for the following gases:- (a) CO2 (b) SO2 (c) O2 (d) H2 Answer:- (a) Carbon dioxide turns lime water milky as it forms calcium carbonate with lime water. This is the characteristics test for carbon dioxide. (b) The smell of sulphur dioxide gas is like burning of sulphur, which is the characteristics test for sulphur dioxide. Sulhpur dioxide turns moist litmus paper from blue to red. (c) When a burnt candle or match stick is brought near the oxygen gas, it starts burning brightly. This is the characteristic test for oxygen gas. (d) When a burnt candle or match stick, it burns with pop sound, which is the characteristic test for hydrogen gas. Question:41. What happens when a piece of (a) zinc metal is added to copper sulphate solution? (b) aluminium metal is added to dilute hydrochloric acid? (c) silver metal is added to copper sulphate solution? Also, write the balanced chemical equation if the reaction occurs Answer: (a) Zinc sulphate and copper is formed when zinc reacts with copper sulphate. (b) Hydrochloric acid forms aluminium chloride and hydrogen gas when reacts with aluminium metal. (c) When silver metal is added to copper sulphate solution, no reaction takes place because silver is less reactive than copper. 42. What happens when zinc granules are treated with dilute solution of H2SO4, HCl, HNO3, NaCl and NaOH, also write the chemical equations if reaction occurs. Answer: Hydrogen gas and zinc sulphate are formed when zinc granules are treated with sulphuric acid. Zinc chloride and hydrogen gas are formed, when zinc granules are treated with hydrochloric acid. Similarly, hydrogen gas and zinc nitrate are formed when zinc granules are treated with nitric acid. Since, zinc is less reactive than sodium, thus no reaction takes place when zinc granules are treated with sodium chloride. Sodium zincate and hydrogen gas are formed when zinc granules are treated with sodium hydroxide and water. 43. On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, white precipitate is obtained. (a) Write a balanced chemical equation of the reaction involved (b) What other name can be given to this precipitation reaction? (c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why? Answer: (a) A white precipitate of barium sulphite is formed when barium chloride is added to the solution o sodium sulphite. (b) This precipitation reaction is also a double displacement reaction. (c) Barium chloride, suphur dioxide and water are formed hen dilute hydrochloric acid is added to this solution of barium sulphete and sodium chloride. Since barium chloride is a soluble substance, thus white precipitate of barium sulphite disappears. 44. You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute HNO3, ZnCl2 and H2O. In which of the above containers these solutions can be kept? Answer: The solution of dilute HCl, dilute HNO3, ZnCl2 and H2O can be kept in the container made of copper instead of aluminium. Since copper is a noble metal and does not react with HCl, HNO3 while these react with aluminium. Thus container made of copper would be suitable to keep the given solutions instead of aluminium. Chemical Reactions and Equations Test: 1- Question 1: What is rust?  (a) Sodium Oxide (b) Iron oxide. (c) Copper Oxide. (d) Silver Oxide.            Question 2:- Because of the formation of which of the following lime water turns milky when carbon dioxide is passed in it? (a) Calcium Carbonate (b) Calcium bicarbonate (c) Calcium hydroxide (d) Sodium carbonate            Question 3: Which of the following is formed when sodium hydroxide reacts with hydrochloric acid? (a) Calcium chloride (b) Hydrogen Chloride (c) Sodium Hydroxide (d) Sodium Chloride            Question 4: Hydrolysis of water is which type of following reactions? (a) Endothermic (b) Decomposition (c) Both (a) and (b) (d) Combination            Question 5: Silver chloride turns grey because of the formation of which of the following when placed in sun? (a) Silver Metal (b) Carbon dioxide (c) Silver Oxide (d) Silver sulphate            Question 6: When sulphuric acid is poured over zinc, which of the following gas is formed? (a) Sulphur dioxide (b) Hydrogen (c) Oxygen (d) Zinc dioxide            Question 7: When copper sulphate solution reacts with iron metal, copper metal is formed. This reaction comes under which of the following category? (a) Decomposition Reaction (b) Single Displacement reaction (c) Double displacement reaction (d) Combination reaction            Question 8: When copper oxide is heated with hydrogen, copper metal and water are formed. What happens to the copper oxide in this reaction? (a) Reduction (b) Oxidation (c) Oxidation & reduction (d) Decomposition            Question 9: When copper oxide is heated with hydrogen, copper metal and water are formed. What happens to the Hydrogen in this reaction? (a) Reduction (b) Oxidation (c) Both oxidation & Reduction (d) Decomposition            Question 10: When copper oxide is heated with hydrogen, copper metal and water are formed. Which of the following is oxidising agent in this reaction? (a) Copper oxide (b) Hydrogen (c) Copper (d) Water Chemical Reactions and Equations Class Test 2 Question: 1 - Which of the following is true for a balanced chemical equation? (a) The number of various atoms is equal in both sides. (b) The number of various atoms is more in left side. (c) The number of various atoms is more in right side. (d) Both (b) and (c).           Question: 2 - - Which of the following is true for an unbalanced chemical equation? (a) The number of various atoms is equal in both sides (b) The number of various atoms is more in left side (c) The number of various atoms is more in right side (d) Both (b) and (c)           Question: 3: - Taking place of a reaction can be shown by which of the following? (a) Change in colour (b) Change in state (c) Change in state (d) All of the above         Question: 4 - Heating of calcium carbonate is which type of following reaction? (a) Combination (b) Decomposition (c) Displacement (d) Double displacement         Question: 5 - Calcium oxide is known by which of the following name? (a) Quick lime (b) Slaked lime (c) Lime stone (d) Marble          Question: 6 - When calcium oxide is treated with water it changes into which of the following? (a) Quick lime (b) Slaked lime (c) Lime stone (d) Marble         Question: 7 - Which of the following is the example of combination reaction? (a) Burning of coal (b) Formation of water (c) Burning of natural gas (d) Both (a) and (b)         Question: 8 - Which of the following is the example of exothermic reaction? (a) Burning of coal (b) Burning of natural gas (c) Reaction during respiration (d) All of the above         Question: 9 -Name the brown fume coming out when lead nitrate is heated? (a) Nitrogen dioxide (b) Nitrogen monoxide (c) Nitrogen (d) Sulphur dioxide         Question: 10 - When iron nail is dipped in copper sulphate solution, why its colour changes to brown? (a) Due to deposition of copper sulphate (b) Due to formation of iron sulphate (c) Due to hydrogen gas (d) All of the above         Acid Base And Salt Acid Acids: The word ‘Acid’ came from Latin word ‘Acidus or Acere’ which means sour. Sour taste is the most common characteristic of acid. Acid turns blue litmus paper red. There are many substances which contain acid and hence taste sour, such as curd, tamarind, lemon, etc. Types of Acids: Acids are divided into two types on the basis of their occurrence – Natural acids and Mineral acids. Natural Acid: Acids which are obtained from natural sources are called natural acid or organic acid. Some common organic acid their and natural source Name of Acid Source Acetic acid Vinegar Ascorbic acid Amla Guava Citric acid Lemon Orange Other citrus food Lactic acid Sour milk Curd Methanoic acid Ant sting Nettle sting Oxalic acid Tomato Tartaric acid Tamarind Mineral Acids:- Acids that prepare from mineral are known as mineral acids, inorganic acids, man-made acids or synthetic acid, such as hydrochloric acid, sulphuric acid, nitric acid, etc. Chemical Properties of Acid Reaction of acids with metal: Acids give hydrogen gas along with respective salt when they react with a metal. Example:- Hydrogen gas and zinc chloride are formed when hydrochloric acid reacts with zinc metal. Hydrogen gas and sodium chloride are formed when hydrochloric acid reacts with sodium metal. Hydrogen gas and iron chloride are formed when hydrochloric acid reacts with iron. Hydrogen gas and zinc sulphate are formed when zinc metal reacts with sulphuric acid Test for hydrogen gas: The gas evolved after reaction of acid with metal can be tested by bringing a lighted candle near it. If the gas burns with pop sound, then it confirms the evolution of hydrogen gas. Burning with pop sound is the characteristic test for hydrogen gas. Reaction of acids with metal carbonate: Acids give carbon dioxide gas and respective salts along with water when they react with metal carbonates. Examples: Hydrochloric acid gives carbon dioxide gas, sodium chloride along with water when reacts with sodium carbonate. Hydrochloric acid gives carbon dioxide, magnesium chloride and water when it reacts with magnesium carbonate. Hydrochloric acid gives carbon dioxide gas, calcium chloride and water when it reacts with calcium carbonate. Sulphuric acid gives calcium sulphate, carbon dioxide gas, calcium sulphate and water when it reacts with calcium carbonate. Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium carbonate. Nitric acid gives sodium nitrate, water and carbon dioxide gas when it reacts with sodium carbonate. Reaction of acid with hydrogen carbonates (bicarbonates): Acids give carbon dioxide gas, respective salt and water when they react with metal hydrogen carbonate. Examples: Hydrochloric acid gives carbon dioxide, sodium chloride and water when it reacts with sodium bicarbonate. Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium bicarbonate. Sodium bicarbonate is also known as sodium hydrogen carbonate, baking soda, baking powder, bread soda and bicarbonate of soda. Test for evolution of carbon dioxide gas: Carbon dioxide turns lime water milky when passed through it. This is the characteristic test for carbon dioxide gas. The gas evolved because of reaction of acid with metal carbonate or metal hydrogen carbonate turns lime water milky. This shows that the gas is carbon dioxide gas. This happens because of formation of white precipitate of calcium carbonate. But when excess of carbon dioxide is passed through lime water, it makes milky colour of lime water disappear. This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is soluble in water, thus the milky colour of solution mixture disappears. Reaction of acid with marble and egg shell: Since, marble and egg shell are made of calcium carbonate, hence when acid is poured over marble or egg shell, bubbles of carbon dioxide are formed. Historical Monuments and Acid Rain: Burning of fossil fuels releases oxides of sulphur and nitrogen. Nitrogen oxide and sulphur dioxide form nitric acid and sulphuric acid on reaction with water. When rain droplets mix with these gases; present in atmosphere because of pollution; they form acid rain. Acid rain causes damage to the historical monuments and other buildings. For example Taj Mahal, which is made of marble, is getting damaged because of reaction with acid rain. Marble is calcium carbonate which reacts with the acid and thus gets corroded. Use of Reaction between Acid and Metal Carbonate or Metal Bicarbonate in Fire Extinguisher: Metal carbonate or metal hydrogen carbonate and acid are used in fire extinguisher to produce carbon dioxide gas. Acid and metal carbonate or bicarbonate are kept in separate chambers in a fire extinguisher. On emergency they are allowed to react with one another. The carbon dioxide gas so produce is poured over fire. As carbon dioxide does not support burning, it puts off the fire. Acid Base and Salt Base Base is bitter in taste and feels soapy on touch. A base turns red litmus paper blue. Sodium hydroxide (caustic soda), calcium hydroxide, sodium carbonate (washing soda), lime (Calcium oxide), potassium hydroxide (caustic potash), etc. are examples of base. Types of base: Base can be divided in two types – Water soluble and water insoluble. The ionic salt of alkali and alkaline earth metals are soluble in water. These are also known as alkali. For example – sodium hydroxide, magnesium hydroxide, calcium hydroxide, etc. Alkali is considered as strong base. Reaction of Base with Metals: When alkali (base) reacts with metal, it produces salt and hydrogen gas. Example:- Sodium hydroxide gives hydrogen gas and sodium zincate when reacts with zinc metal. Sodium aluminate and hydrogen gas are formed when sodium hydroxide reacts with aluminium metal. Reaction of base with oxides of non-metals: Non-metal oxides are acidic in nature. For example; carbon dioxide is a non-metal oxide. When carbon dioxide is dissolved in water it produces carbonic acid. Therefore, when a base reacts with non-metal oxide both neutralize each other resulting respective salt and water are produced. Example :- Sodium hydroxide gives sodium carbonate and water when it reacts with carbon dioxide. Calcium hydroxide gives calcium carbonate and water when it reacts with carbon dioxide. Reaction of an Acid with a Base - Neutralisation Reaction: An acid neutralizes a base when they react with each other and respective salt and water are formed. Since in the reaction between acid and base both neutralize each other, hence it is also known as neutralization reaction. Example:- Sodium chloride and water are formed when hydrochloric acid reacts with sodium hydroxide (a strong base). In similar way, calcium chloride is formed along with water when hydrochloric acid reacts with calcium hydroxide (a base). Sodium sulphate and water are formed when sulphuric acid reacts with sodium hydroxide (a base). In similar way, when nitric acid reacts with sodium hydroxide, sodium nitrate and water are formed. Reaction of Acid with Metal Oxides: Metal oxides are basic in nature. Thus, when an acid reacts with a metal oxide both neutralize each other. In this reaction, respective salt and water are formed. Example: Calcium is a metal, thus calcium oxide is a metallic oxide which is basic in nature. When an acid; such as hydrochloric acid; reacts with calcium oxide, neutralization reaction takes place and calcium chloride; along with water; is formed. Similarly, when sulphuric acid reacts with zinc oxide, zinc sulphate and water are formed. When hydrochloric acid reacts with aluminium oxide, aluminium chloride and water are formed. Acid Base and Salt Common in Acid and Base Acids give hydrogen gas when they react with metal. This shows that all acids contain hydrogen. For example: Hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3), etc. When an acid is dissolved in water, it dissociates hydrogen. The dissociation of hydrogen ion in aqueous solution is the common property in all acids. Because of dissociation of hydrogen ion in aqueous solution, an acid shows acidic behavior. Example: Hydrochloric acid (HCl) gives hydrogen ion (H+) and chloride ion (Cl-) when it is dissolved in water. Sulphuric acid (H2 SO4) gives hydrogen ion (H+) and sulphate ion (SO4--) in water. Nitric acid (HNO3) gives hydrogen ion (H+) and nitrate ion (NO3-) in water. Acetic acid (CH3COOH) gives acetate ion (CH3COO-) and hydrogen ion (H+). Hydrogen ion which is produced by acid (when acid is combined with water molecule), exists in the form of hydronium ion (H3O-) in aqueous solution. That’s why hydrogen ion is always written with suffix (aq), such as H+ (aq). Example: Thus, because of dissociation of hydrogen ions; acid shows its acidic behavior. Acids conduct electricity in their aqueous solution because of dissociation of hydrogen ion. Hydrogen ion in aqueous solution conducts electricity. A dry acid, such as dry hydrochloric acid does not change the colour of blue litmus paper to red because a dry acid does not dissociate hydrogen ion. This is the cause that a moist litmus paper is used to check the acidic or basic character of a gas. Acidic behavior of carbon dioxide gas: Carbon dioxide gas produces carbonic acid when dissolved in water. This carbonic acid dissociates hydrogen ion and carbonate ion in the aqueous solution. Are all compounds which contain hydrogen, necessarily acids? No, all compounds which contain hydrogen are not acid. For example; glucose (C6H12O6), methyl alcohol (CH3OH), etc. are not acid in spite of the fact that they contain hydrogen. This is because these compounds do not dissociate hydrogen ion in their aqueous solution. Common in all Base: A base dissociates hydroxide ion in water, which is responsible for the basic behavior of a compound. Example: When sodium hydroxide is dissolved in water, it dissociates hydroxide ion and sodium ion. Similarly, when potassium hydroxide is dissolved in water, it dissociates hydroxide ion and potassium ion. Thus, base shows its basic character because of dissociation of hydroxide ion. Neutralisation Reaction: When an acid reacts with a base, the hydrogen ion of acid combines with the hydroxide ion of base and forms water. As these ions combine together and form water; instead of remaining free, thus both neutralize each other. Example:- When sodium hydroxide (a base) reacts with hydrochloric acid, sodium hydroxide breaks into sodium ion and hydroxide ion and hydrochloric acid breaks into hydrogen ion and chloride ion. Hydrogen ion and hydroxide ion combine together and form water, while sodium ion and chloride ion combine together and form sodium chloride. Dilution of Acid and Base: The concentration of hydrogen ion in an acid and hydroxide ion in a base; per unit volume; shows the concentration of acid or base. By mixing of acid to water, the concentration of hydrogen ion per unit volume decreases. Similarly, by addition of base to water the concentration of hydroxide ion per unit volume decreases. This process of addition of acid or base to water is called dilution and the acid or base is called diluted. The dilution of acid or base is exothermic. Thus, acid or base is always added to water and water is never added to acid or base. If water is added to a concentrated acid or base a lot of heat is generated, which may cause splashing out of acid or base and may cause severe damage as concentrated acid and base are highly corrosive. Acid Base and Salt Strength of Acid and Base: Acids in which complete dissociation of hydrogen ion takes place are called strong acid. Similarly, bases in which complete dissociation of hydroxide ion takes place are called strong base. In mineral acids, such as hydrochloric acid, sulphuric acid, nitric acid, etc. hydrogen ion dissociates completely and hence they are considered as strong acid. Since, in organic acids hydrogen ions do not dissociate completely, so they are weak acid. Alkalis are water soluble base, thus in alkali; complete dissociation of hydroxide ions takes place and they are considered as strong base. The complete dissociation of hydrogen ions or hydroxide ions is shown by a single arrow. The incomplete dissociation of hydrogen ions or hydroxide ions is denoted by double arrow. Example of complete dissociation: Example of incomplete dissociation:- Although acetic acid being an organic acid is a weak acid, but concentrated acetic acid is corrosive and can damage the skin if poured over it. pH – Measurement of strength of Acid and Base The strength of acid or base depends upon the hydrogen ion concentration. If the concentration of hydrogen ion is greater than hydroxide ion, the solution is called acidic. If the concentration of hydrogen ion is smaller than the hydroxide ion, the solution is called basic. If the concentration of hydrogen ion is equal to the concentration of hydroxide ion, the solution is called neutral solution. pH is a scale which quantifies the concentration of hydrogen ion in a solution. The range of pH scale is between 0 to 14. The pH value decreases with increase in hydrogen ion concentration. If the value of pH is 0, this shows maximum hydrogen ion concentration. pH value equal to 14 shows lowest hydrogen ion concentration. pH value equal to 7 shows the hydrogen ion concentration is equal to hydroxide ion concentration. A neutral solution, such as distilled water has value of hydrogen ion concentration equal to 7 on pH scale. The acidic solution has value of hydrogen ion concentration less than 7 on pH scale. The basic solution has value of hydrogen ion concentration greater than 7 on pH scale. In pH scale ‘p’ stands for ‘potenz’. Potenz is a German word which means ‘power’ or ‘potential’. Here; ‘H’ stands for hydrogen ion. Thus, pH means the potential of hydrogen or power of hydrogen. pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity (a_(H^+ )), in a solution.   Indicator: Substances which show the acidic or basic behavior of other substance by change in colour are known as indicator. Type of Indicator: There are many types of indicators. Some common types of indicators are Natural Olfactory Indicator Synthetic Indicator Universal Indicator Natural Indicator:- Indicators obtained from natural sources are called natural indicators. Litmus, turmeric, red cabbage, China rose, etc. are some common natural indicators used widely to show the acidic or basic character of substances. Litmus: Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colour – blue and red. An acid turns blue litmus paper red. A base turns red litmus paper blue. Turmeric:- Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid. Red cabbage: The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base. Olfactory Indicators:- Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example onion, vanilla, clove, etc. Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid. Vanilla: The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid. Olfactory indicators are used to ensure the participation of visually impaired students in laboratory. Synthetic Indicator: Indicators that are synthesized in laboratory are known as synthetic indicators. For example; phenolphthalein, methyl orange, etc. Phenolphthalein is a colourless liquid. It remains colourless with acid but turns into pink with a base. Methyl orange is originally orange in colour. It turns into red with acid and turns into yellow with base. Universal Indicator:- Using a litmus paper, phelophthalein, methyl orange, etc. only the acidic or basic character of a solution can be determined, but use of these indicators does not give the idea about the strength of acid or base. So, to get the strength as well as acidic and basic nature of a given solution universal indicator is used. Universal indicator shows different colour over the range of pH value from 1 to 14 for a given solution. Universal indicator is available both in the form of strips and solution. Universal indicator is the combination of many indicators, such as water, propanol, phelophthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt. The colour matching chart is supplied with universal indicator which shows the different colours for different values of pH. Colour of Universal Indicator for different value of pH scale. Acid Base and Salt Salt Salts are the ionic compounds which are produced after the neutralization reaction between acid and base. Salts are electrically neutral. There are number of salts but sodium chloride is the most common among them. Sodium chloride is also known as table salt or common salt. Sodium chloride is used to enhance the taste of food. Family of Salt: Salts having common acidic or basic radicals are said to belong to same family. Example:- Sodium chloride (NaCl) and Calcium chloride (CaCl2) belong to chloride family. Calcium chloride (CaCl2) and calcium sulphate (CaSO4) belong to calcium family. Zinc chloride (ZnCl2) and Zinc sulphate (ZnSO4) belong to zinc family. Acidic, Basic and Neutral Salt:- Neutral Salt: Salts produced because of reaction between strong acid and strong base are neutral in nature. The pH of value of such salts is equal to 7, i.e. neutral. Example; Sodium chloride, sodium sulphate, potassium chloride, etc. Sodium chloride (NaCl) is formed after the reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base). Sodium sulphate (Na2SO4): It is formed after the reaction between sodium hydroxide (a strong base) and sulphuric acid (a strong acid). Potassium chloride (KCl): It is formed after the reaction between potassium hydroxide (a strong base) and hydrochloric acid (a strong acid). Acidic salt:- Salts which are formed after the reaction between a strong acid and weak base are called acidic salt. The pH value of acidic salt is lower than 7. Example; ammonium sulphate, ammonium chloride, etc. Ammonium chloride is formed after reaction between hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base). Ammonium sulphate is formed after reaction between ammonium hydroxide (weak base) and sulphuric acid (a strong acid). Basic Salt:- Salts which are formed after the reaction between weak acid and strong base are called basic salt. For example; sodium carbonate, sodium acetate, etc. Sodium carbonate is formed after the reaction between sodium hydroxide (a strong base) and carbonic acid (a weak base). Sodium acetate is formed after the reaction between a strong base, sodium hydroxide and a weak acid, acetic acid. Cause of formation of acidic, basic and neutral salt:- When a strong acid reacts with a weak base, the base is unable to fully neutralize the acid. Due to this an acidic salt is formed in this case. When a strong base reacts with a weak acid, the acid is unable to fully neutralize the base. Due to this a basic salt is formed in this case. When equally strong acid and base react they fully neutralize each other. Due to this a neutral salt is formed in this case. pH value of salt:- Neutral salt: The pH value of a neutral salt is almost equal to 7. Acidic salt: The pH value of an acidic salt is less than 7. Basic salt: The pH value of a basic salt is more than 7. Characteristics of salt: Most of the salts are crystalline solid. Salts may be transparent or opaque. Most of the salts are soluble in water. Solution of salts conducts electricity. Salts conduct electricity in their molten state also. The salt may be salty, sour, sweet, bitter and umami (savoury). Neutral salts are odourless. Salts can be colourless or of coloured. Acid Base and Salt Common Salt (Sodium Chloride) Sodium chloride (NaCl) is also known as common or table salt. It is formed after the reaction between sodium hydroxide and hydrochloric acid. It is a neutral salt. The pH value of sodium chloride is about 7. Sodium chloride is used to enhance the taste of food. Sodium chloride is used in manufacturing of many chemicals. Important chemicals from sodium chloride:- Sodium Hydroxide (NaOH):- Sodium hydroxide is a strong base. It is also known as caustic soda or Iye. It is obtained by the electrolytic decomposition of solution of sodium chloride (brine). In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide. In this process, chlorine is obtained at anode and hydrogen gas is obtained at cathode as byproducts. This whole process is known as Chlor-Alkali process. Use of products after the electrolysis of brine:- Hydrogen gas is used as fuel, margarine, in making of ammonia for fertilizer, etc. Chlorine gas is used in water treatment, manufacturing of PVC, disinfectants, CFC, pesticides. It is also used in manufacturing of bleaching powder and hydrochloric acid. Sodium hydroxide: De-greasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc. Bleaching Powder (CaOCl2):- Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine. When calcium hydroxide (slaked lime) reacts with chlorine, it gives calcium oxychloride (bleaching powder) and water is formed. Water solution of bleaching powder is basic in nature. The term bleach means removal of colour. Bleaching powder is often used as bleaching agent. It works because of oxidation. Chlorine in the bleaching powder is responsible for bleaching effect. Use of bleaching powder:- Bleaching powder is used as disinfectant to clean water, moss remover, weed killers, etc. Bleaching powder is used for bleaching of cotton in textile industry, bleaching of wood pulp in paper industry. Bleaching powder is used as oxidizing agent in many industries, such as textiles industry, paper industry, etc. Baking Soda (NaHCO3) Baking soda is another important product which can be obtained using byproducts of chlor-alkali process. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO3) or sodium bicarbonate. Bread soda, cooking soda, bicarbonate of soda, sodium bicarb, bicarb of soda or simply bicarb, etc. are some other names of baking soda. Baking soda is obtained by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process. In this process, calcium carbonate is used as the source of CO2 and the resultant calcium oxide is used to recover ammonia from ammonium chloride. Properties of sodium bicarbonate: Sodium bicarbonate is white crystalline solid, but it appears as fine powder. Sodium hydrogen carbonate is amphoteric in nature. Sodium hydrogen carbonate is sparingly soluble in water. Thermal decomposition of sodium hydrogen carbonate (baking soda) When baking soda is heated, it decomposes into sodium carbonate, carbon dioxide and water. Sodium carbonate formed after thermal decomposition of sodium hydrogen carbonate; decomposes into sodium oxide and carbon dioxide on further heating. This reaction is known as dehydration reaction. Use of Baking Soda: Baking soda is used in making of baking powder, which is used in cooking as it produces carbon dioxide which makes the batter soft and spongy. Baking soda is used as antacid. Baking soda is used in toothpaste which makes the teeth white and plaque free. Baking soda is used in cleansing of ornaments made of sliver. Since, sodium hydrogen carbonate gives carbon dioxide and sodium oxide on strong heating, thus it is used as fire extinguisher. Baking powder: Baking powder produces carbon dioxide on heating, so it is used in cooking to make the batter spongy. Although baking soda also produces carbon dioxide on heating, but it is not used in cooking because on heating; baking soda produces sodium carbonate along with carbon dioxide. The sodium carbonate; thus produced; makes the taste bitter. Baking powder is the mixture of baking soda and a mild edible acid. Generally, tartaric acid is mixed with baking soda to make baking powder. When baking powder (mixture of baking soda and an edible acid) is heated, the sodium carbonate formed because of heating of baking soda neutralizes after reacting with tartaric acid and sodium tartarate salt is formed. The smell of sodium tartarate is pleasant and taste is good. This makes the cake or any other food tasty. Washing Soda (Sodium carbonate) Sodium carbonate is manufactured by the thermal decomposition of sodium hydrogen carbonate obtained by Solvay process. The sodium carbonate obtained in this process is dry. It is called soda ash or anhydrous sodium carbonate. Washing soda is obtained by rehydration of anhydrous sodium carbonate. Since there are 10 water molecules in washing soda, hence it is known as Sodium bicarbonate decahydrate. Sodium carbonate is a crystalline solid and it is soluble in water when most of the carbonates are insoluble in water. Acid Base and Salts Exemplar Problem and Solution (NCERT) MCQs (Multiple Choice Questions - Exemplar Problem and solution) Part - I Question: 1. What happens when a solution of an acid is mixed with a solution of a base in a test tube? (i) The temperature of the solution increases. (ii) The temperature of the solution decreases. (iii) The temperature of the solution remains the same. (iv) Salt formation takes place (a) (i) only (b) (i) and (iii) (c) (ii) and (iii) (d) (i) and (iv) Answer : (d) (i) and (iv) Question: 2. An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change? (a) Baking powder (b) Lime (c) Ammonium hydroxide solution (d) Hydrochloric acid Answer:- (d) Hydrochloric acid Question: 3. During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to (a) absorb the evolved gas (b) moisten the gas (c) absorb moisture from the gas (d) absorb Cl– ions from the evolved gas Answer:- (c) absorb moisture from the gas Question: 4. Which of the following salts does not contain water of crystallisation? (a) Blue vitriol (b) Baking soda (c) Washing soda (d) Gypsum Answer: (b) Baking soda Question: 5. Sodium carbonate is a basic salt because it is a salt of (a) strong acid and strong base (b) weak acid and weak base (c) strong acid and weak base (d) weak acid and strong base Answer: (d) Weak acid and strong base Question: 6. Calcium phosphate is present in tooth enamel. Its nature is (a) basic (b) acidic (c) neutral (d) amphoteric Answer: (c) neutral Question: 7. A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this pH paper to greenish-blue? (a) Lemon juice (b) Vinegar (c) Common salt (d) An antacid Answer: (d) An antacid 8. Which of the following gives the correct increasing order of acidic strength? (a) Water < Acetic acid < Hydrochloric acid (b) Water < Hydrochloric acid < Acetic acid (c) Acetic acid < Water < Hydrochloric acid (d) Hydrochloric acid < Water < Acetic acid Answer: (a) Water < Acetic acid < Hydrochloric acid 9. If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done? (a) Wash the hand with saline solution (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogencarbonate (c) After washing with plenty of water apply solution of sodium hydroxide on the hand (d) Neutralise the acid with a strong alkali Answer: (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogen carbonate. 10. Sodium hydrogencarbonate when added to acetic acid evolves a gas. Which of the following statements are true about the gas evolved? (i) It turns lime water milky (ii) It extinguishes a burning splinter (iii) It dissolves in a solution of sodium hydroxide (iv) It has a pungent odour (a) (i) and (ii) (b) (i), (ii) and (iii) (c) (ii), (iii) and (iv) (d) (i) and (iv) Answer: (a) (i) and (ii)  MCQs - Part -II (exemplar problem and solution)  Acid Base and Salts Exemplar Problem and Solution (NCERT) MCQs (Multiple Choice Questions - Exemplar Problem and solution) Part - II 11. Common salt besides being used in kitchen can also be used as the raw material for making soda (ii) bleaching powder (iii) baking soda (iv) slaked lime (a) (i) and (ii) (b) (i), (ii) and (iv) (c) (i) and (iii) (d) (i), (iii) and (iv) Answer: (c) (i) and (iii) 12. One of the constituents of baking powder is sodium hydrogencarbonate, the other constituent is (a) hydrochloric acid (b) tartaric acid (c) acetic acid (d) sulphuric acid Answer: (b) Tartaric acid 13. To protect tooth decay we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is (a) acidic (b) neutral (c) basic (d) corrosive Answer: (c) basic Question: 14. Which of the following statements is correct about an aqueous solution of an acid and of a base? (i) Higher the pH, stronger the acid (ii) Higher the pH, weaker the acid (iii) Lower the pH, stronger the base (iv) Lower the pH, weaker the base (a) (i) and (iii) (b) (ii) and (iii) (c) (i) and (iv) (d) (ii) and (iv) Answer: (c) (i) and (iv) Question: 15. The pH of the gastric juices released during digestion is (a) less than 7 (b) more than 7 (c) equal to 7 (d) equal to 0 Answer: (a) less than 7 Question: 16. Which of the following phenomena occur, when a small amount of acid is added to water? (i) Ionisation (ii) Neutralisation (iii) Dilution (iv) Salt formation (a) (i) and (ii) (b) (i) and (iii) (c) (ii) and (iii) (d) (ii) and (iv) Answer: (b) (i) and (iii) Question: 17. Which one of the following can be used as an acid–base indicator by a visually impared student? (a) Litmus (b) Turmeric (c) Vanilla essence (d) Petunia leaves Answer: (c) Vanilla essence Question: 18. Which of the following substance will not give carbon dioxide on treatment with dilute acid? (a) Marble (b) Limestone (c) Baking soda (d) Lime Anwer (d) Lime Question: 19. Which of the following is acidic in nature? (a) Lime juice (b) Human blood (c) Lime water (d) Antacid AnswerAnswer: (b) Human blood Question: 20. In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (Given Figure) was set up. Which among the following statement(s) is(are) correct? (i) Bulb will not glow because electrolyte is not acidic (ii) Bulb will glow because NaOH is a strong base and furnishes ions for conduction. (iii) Bulb will not glow because circuit is incomplete (iv) Bulb will not glow because it depends upon the type of electrolytic solution (a) (i) and (iii) (b) (ii) and (iv) (c) (ii) only (c) (iv) only Answer: (c) (ii) only Acid Base and Salts Exemplar Problem and Solution (NCERT) MCQs (Multiple Choice Questions - Exemplar Problem and solution) Part - III Question: 21. Which of the following is used for dissolution of gold? (a) Hydrochloric acid (b) Sulphuric acid (c) Nitric acid (d) Aqua regia Answer:- (d) Aqua regia Question: 22. Which of the following is not a mineral acid? (a) Hydrochloric acid (b) Citric acid (c) Sulphuric acid (d) Nitric acid Answer: (b) Citric acid Question: 23. Which among the following is not a base? (a) NaOH (b) KOH (c) NH4OH (d) C2H5 OH Answer: (d) C2H5 OH Question: 24. Which of the following statements is not correct? (a) All metal carbonates react with acid to give a salt, water and carbon dioxide (b) All metal oxides react with water to give salt and acid (c) Some metals react with acids to give salt and hydrogen (d) Some non metal oxides react with water to form an acid Answer: (b) All metal oxides react with water to give salt and acid. Question: 25. Match the chemical substances given in Column (A) with their appropriate application given in Column A Column B (A) Bleaching Powder (i) Preparation of Glass (B) Baking Soda (ii) Production of H2 and Cl2 (C) Washing Soda (iii) Decolourisation (D) Sodium Chloride (iv) Antacid (a) A—(ii), B—(i), C—(iv), D—(iii) (b) A—(iii), B—(ii), C—(iv), D—(i) (c) A—(iii), B—(iv), C—(i), D—(ii) (d) A—(ii), B—(iv), C—(i), D—(iii) Answer: (c) A – (iii), B – (iv), C – (i), D – (ii) Question: 26. Equal volumes of hydrochloric acid and sodium hydroxide solutions of same concentration are mixed and the pH of the resulting solution is checked with a pH paper. What would be the colour obtained? (You may use colour guide given in given Figure (a) Red (b) Yellow (c) Yellowish green (d) Blue Answer: (c) Yellowish green 27. Which of the following is(are) true when HCl (g) is passed through water? (i) It does not ionise in the solution as it is a covalent compound. (ii) It ionises in the solution (iii) It gives both hydrogen and hydroxyl ion in the solution (iv) It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule (a) (i) only (b) (iii) only (c) (ii) and (iv) (d) (iii) and (iv) Answer: (c) (ii) and (iv) 28. Which of the following statements is true for acids? (a) Bitter and change red litmus to blue (b) Sour and change red litmus to blue (c) Sour and change blue litmus to red (d) Bitter and change blue litmus to red Answer: (c) Sour and change blue litmus to red 29. Which of the following are present in a dilute aqueous solution of hydrochloric acid? (a) H3O+ + Cl– (b) H3O+ + OH– (c) Cl– + OH– (d) unionised HCl Answer (a) H3O+ + Cl– 30. Identify the correct representation of reaction occurring during chloralkali process (a) 2NaCl(l) + 2H2O(l) → 2NaOH(l) + Cl2(g) + H2(g) (b) 2NaCl(aq) + 2H2O(aq) → 2NaOH(aq) + Cl2(g) + H2(g) (c) 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(aq) + H2(aq) (d) 2NaCl (aq) + 2H2O (l) → 2NaOH (aq) + Cl2(g) + H2(g) Answer: (d) 2NaCl (aq) + 2H2O (l) → 2NaOH (aq) + Cl2(g) + H2(g) Acid Base and Salt Exemplar Quesions (NCERT) and Answer Short Answer Type - I Exemplar Problem: 31 - Match the acids given in Column (A) with their correct source given Exemplar Problem: 32. Match the important chemicals given in Column (A) with the chemical formulae given in Column (B) Exemplar Problem: 33. What will be the action of the following substances on litmus paper? Dry HCl gas, Moistened NH3 gas, Lemon juice, Carbonated soft drink, Curd, Soap solution. Answer: Dry HCl – No action Moistened NH3 gas – Turns blue litmus to red Lemon Juice – Turns blue litmus to red Carbonated soft drink – Turns blue litmus to red. Curd – Turns blue litmus to red Soap solution – Turns red litmus to blue. Explanation:- (a) Dry HCl does not liberate hydrogen ion, hence no action takes place with litmus paper. (b) NH3 gas forms ammonium hydroxide in with water which turns red litmus paper blue. (c) Lemon juice is citric acid, so it turns blue litmus paper red. (d) Carbonate soft drink contains carbon dioxide dissolved in water. Carbon dioxide forms carbonic acid; with water; which turns blue litmus paper red. (e) Curd contains lactic acid and hence turns blue litmus paper to red. (f) Soap solution is basic in nature hence it turns red litmus paper to blue. Exemplar Problem:- 34. Name the acid present in ant sting and give its chemical formula. Also give the common method to get relief from the discomfort caused by the ant sting. Answer:- Name of acid present in ant sting: Methanoic acid Chemical Formula of methanoic acid: HCOOH Method to get relief from the discomfort caused by the ant sting: Rubbing baking soda over the location of ant sting. Explanation: Since baking soda is a base thus by rubbing over the ant sting it neutralizes the methanoic acid present in ant sting and gives relief from pain. Exemplar Problem: 35. What happens when nitric acid is added to egg shell? Answer:- Egg shell is made of calcium carbonate. We know that when calcium carbonate reacts with nitric acid, it gives carbon dioxide, calcium nitrate and water. Same reaction happens when nitric acid is poured over egg shell. ' Acid Base and Salt Salt Salts are the ionic compounds which are produced after the neutralization reaction between acid and base. Salts are electrically neutral. There are number of salts but sodium chloride is the most common among them. Sodium chloride is also known as table salt or common salt. Sodium chloride is used to enhance the taste of food. Family of Salt: Salts having common acidic or basic radicals are said to belong to same family. Example:- Sodium chloride (NaCl) and Calcium chloride (CaCl2) belong to chloride family. Calcium chloride (CaCl2) and calcium sulphate (CaSO4) belong to calcium family. Zinc chloride (ZnCl2) and Zinc sulphate (ZnSO4) belong to zinc family. Acidic, Basic and Neutral Salt:- Neutral Salt: Salts produced because of reaction between strong acid and strong base are neutral in nature. The pH of value of such salts is equal to 7, i.e. neutral. Example; Sodium chloride, sodium sulphate, potassium chloride, etc. Sodium chloride (NaCl) is formed after the reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base). Sodium sulphate (Na2SO4): It is formed after the reaction between sodium hydroxide (a strong base) and sulphuric acid (a strong acid). Potassium chloride (KCl): It is formed after the reaction between potassium hydroxide (a strong base) and hydrochloric acid (a strong acid). Acidic salt:- Salts which are formed after the reaction between a strong acid and weak base are called acidic salt. The pH value of acidic salt is lower than 7. Example; ammonium sulphate, ammonium chloride, etc. Ammonium chloride is formed after reaction between hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base). Ammonium sulphate is formed after reaction between ammonium hydroxide (weak base) and sulphuric acid (a strong acid). Basic Salt:- Salts which are formed after the reaction between weak acid and strong base are called basic salt. For example; sodium carbonate, sodium acetate, etc. Sodium carbonate is formed after the reaction between sodium hydroxide (a strong base) and carbonic acid (a weak base). Sodium acetate is formed after the reaction between a strong base, sodium hydroxide and a weak acid, acetic acid. Cause of formation of acidic, basic and neutral salt:- When a strong acid reacts with a weak base, the base is unable to fully neutralize the acid. Due to this an acidic salt is formed in this case. When a strong base reacts with a weak acid, the acid is unable to fully neutralize the base. Due to this a basic salt is formed in this case. When equally strong acid and base react they fully neutralize each other. Due to this a neutral salt is formed in this case. pH value of salt:- Neutral salt: The pH value of a neutral salt is almost equal to 7. Acidic salt: The pH value of an acidic salt is less than 7. Basic salt: The pH value of a basic salt is more than 7. Characteristics of salt: Most of the salts are crystalline solid. Salts may be transparent or opaque. Most of the salts are soluble in water. Solution of salts conducts electricity. Salts conduct electricity in their molten state also. The salt may be salty, sour, sweet, bitter and umami (savoury). Neutral salts are odourless. Salts can be colourless or of coloured. Acid Base and Salt Common Salt (Sodium Chloride) Sodium chloride (NaCl) is also known as common or table salt. It is formed after the reaction between sodium hydroxide and hydrochloric acid. It is a neutral salt. The pH value of sodium chloride is about 7. Sodium chloride is used to enhance the taste of food. Sodium chloride is used in manufacturing of many chemicals. Important chemicals from sodium chloride:- Sodium Hydroxide (NaOH):- Sodium hydroxide is a strong base. It is also known as caustic soda or Iye. It is obtained by the electrolytic decomposition of solution of sodium chloride (brine). In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide. In this process, chlorine is obtained at anode and hydrogen gas is obtained at cathode as byproducts. This whole process is known as Chlor-Alkali process. Use of products after the electrolysis of brine:- Hydrogen gas is used as fuel, margarine, in making of ammonia for fertilizer, etc. Chlorine gas is used in water treatment, manufacturing of PVC, disinfectants, CFC, pesticides. It is also used in manufacturing of bleaching powder and hydrochloric acid. Sodium hydroxide: De-greasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc. Bleaching Powder (CaOCl2):- Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine. When calcium hydroxide (slaked lime) reacts with chlorine, it gives calcium oxychloride (bleaching powder) and water is formed. Water solution of bleaching powder is basic in nature. The term bleach means removal of colour. Bleaching powder is often used as bleaching agent. It works because of oxidation. Chlorine in the bleaching powder is responsible for bleaching effect. Use of bleaching powder:- Bleaching powder is used as disinfectant to clean water, moss remover, weed killers, etc. Bleaching powder is used for bleaching of cotton in textile industry, bleaching of wood pulp in paper industry. Bleaching powder is used as oxidizing agent in many industries, such as textiles industry, paper industry, etc. Baking Soda (NaHCO3) Baking soda is another important product which can be obtained using byproducts of chlor-alkali process. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO3) or sodium bicarbonate. Bread soda, cooking soda, bicarbonate of soda, sodium bicarb, bicarb of soda or simply bicarb, etc. are some other names of baking soda. Baking soda is obtained by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process. In this process, calcium carbonate is used as the source of CO2 and the resultant calcium oxide is used to recover ammonia from ammonium chloride. Properties of sodium bicarbonate: Sodium bicarbonate is white crystalline solid, but it appears as fine powder. Sodium hydrogen carbonate is amphoteric in nature. Sodium hydrogen carbonate is sparingly soluble in water. Thermal decomposition of sodium hydrogen carbonate (baking soda) When baking soda is heated, it decomposes into sodium carbonate, carbon dioxide and water. Sodium carbonate formed after thermal decomposition of sodium hydrogen carbonate; decomposes into sodium oxide and carbon dioxide on further heating. This reaction is known as dehydration reaction. Use of Baking Soda: Baking soda is used in making of baking powder, which is used in cooking as it produces carbon dioxide which makes the batter soft and spongy. Baking soda is used as antacid. Baking soda is used in toothpaste which makes the teeth white and plaque free. Baking soda is used in cleansing of ornaments made of sliver. Since, sodium hydrogen carbonate gives carbon dioxide and sodium oxide on strong heating, thus it is used as fire extinguisher. Baking powder: Baking powder produces carbon dioxide on heating, so it is used in cooking to make the batter spongy. Although baking soda also produces carbon dioxide on heating, but it is not used in cooking because on heating; baking soda produces sodium carbonate along with carbon dioxide. The sodium carbonate; thus produced; makes the taste bitter. Baking powder is the mixture of baking soda and a mild edible acid. Generally, tartaric acid is mixed with baking soda to make baking powder. When baking powder (mixture of baking soda and an edible acid) is heated, the sodium carbonate formed because of heating of baking soda neutralizes after reacting with tartaric acid and sodium tartarate salt is formed. The smell of sodium tartarate is pleasant and taste is good. This makes the cake or any other food tasty. Washing Soda (Sodium carbonate) Sodium carbonate is manufactured by the thermal decomposition of sodium hydrogen carbonate obtained by Solvay process. The sodium carbonate obtained in this process is dry. It is called soda ash or anhydrous sodium carbonate. Washing soda is obtained by rehydration of anhydrous sodium carbonate. Since there are 10 water molecules in washing soda, hence it is known as Sodium bicarbonate decahydrate. Sodium carbonate is a crystalline solid and it is soluble in water when most of the carbonates are insoluble in water. Use of sodium carbonate: It is used in cleaning of cloths; especially in rural areas. In making of detergent cake and powder. In removing permanent hardness of water. It is used in glass and paper industries. Water of crystallization Many salts contain water molecule and are known as hydrated salts. The water molecule present in salt is known as water of crystallization. Example: Copper sulphate pentahydrate (CuSO4.5H2O): Blue colour of copper sulphate is due to presence of 5 molecules of water. When copper sulphate is heated, it loses water molecules and turns into grey-white colour, which is known as anhydrous copper sulphate. After adding water; anhydrous copper sulphate becomes blue again. Ferrous Sulphate heptahydrate (FeSO4.7H2O): The green colour of Ferrous sulphate heptahydrate; commonly known as ferrous sulphate; is due to the presence of 7 molecules of water in it. Plaster of Paris Plaster of Paris is obtained by heating of gypsum, a hydrated salt of calcium. After addition of water Plaster of Paris is again converted into gypsum. Plaster of Paris is used in making of toys, designer false ceiling, etc. Doctors use Plaster of Paris to set the fractured bone. Acid Base and Salts Exemplar Problem and Solution (NCERT) MCQs (Multiple Choice Questions - Exemplar Problem and solution) Part - I Question: 1. What happens when a solution of an acid is mixed with a solution of a base in a test tube? (i) The temperature of the solution increases. (ii) The temperature of the solution decreases. (iii) The temperature of the solution remains the same. (iv) Salt formation takes place (a) (i) only (b) (i) and (iii) (c) (ii) and (iii) (d) (i) and (iv) Answer : (d) (i) and (iv) Question: 2. An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change? (a) Baking powder (b) Lime (c) Ammonium hydroxide solution (d) Hydrochloric acid Answer:- (d) Hydrochloric acid Question: 3. During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to (a) absorb the evolved gas (b) moisten the gas (c) absorb moisture from the gas (d) absorb Cl– ions from the evolved gas Answer:- (c) absorb moisture from the gas Question: 4. Which of the following salts does not contain water of crystallisation? (a) Blue vitriol (b) Baking soda (c) Washing soda (d) Gypsum Answer: (b) Baking soda Question: 5. Sodium carbonate is a basic salt because it is a salt of (a) strong acid and strong base (b) weak acid and weak base (c) strong acid and weak base (d) weak acid and strong base Answer: (d) Weak acid and strong base Question: 6. Calcium phosphate is present in tooth enamel. Its nature is (a) basic (b) acidic (c) neutral (d) amphoteric Answer: (c) neutral Question: 7. A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this pH paper to greenish-blue? (a) Lemon juice (b) Vinegar (c) Common salt (d) An antacid Answer: (d) An antacid 8. Which of the following gives the correct increasing order of acidic strength? (a) Water < Acetic acid < Hydrochloric acid (b) Water < Hydrochloric acid < Acetic acid (c) Acetic acid < Water < Hydrochloric acid (d) Hydrochloric acid < Water < Acetic acid Answer: (a) Water < Acetic acid < Hydrochloric acid 9. If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done? (a) Wash the hand with saline solution (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogencarbonate (c) After washing with plenty of water apply solution of sodium hydroxide on the hand (d) Neutralise the acid with a strong alkali Answer: (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogen carbonate. 10. Sodium hydrogencarbonate when added to acetic acid evolves a gas. Which of the following statements are true about the gas evolved? (i) It turns lime water milky (ii) It extinguishes a burning splinter (iii) It dissolves in a solution of sodium hydroxide (iv) It has a pungent odour (a) (i) and (ii) (b) (i), (ii) and (iii) (c) (ii), (iii) and (iv) (d) (i) and (iv) Answer: (a) (i) and (ii) Acid Base and Salts Exemplar Problem and Solution (NCERT) MCQs (Multiple Choice Questions - Exemplar Problem and solution) Part - II 11. Common salt besides being used in kitchen can also be used as the raw material for making soda (ii) bleaching powder (iii) baking soda (iv) slaked lime (a) (i) and (ii) (b) (i), (ii) and (iv) (c) (i) and (iii) (d) (i), (iii) and (iv) Answer: (c) (i) and (iii) 12. One of the constituents of baking powder is sodium hydrogencarbonate, the other constituent is (a) hydrochloric acid (b) tartaric acid (c) acetic acid (d) sulphuric acid Answer: (b) Tartaric acid 13. To protect tooth decay we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is (a) acidic (b) neutral (c) basic (d) corrosive Answer: (c) basic Question: 14. Which of the following statements is correct about an aqueous solution of an acid and of a base? (i) Higher the pH, stronger the acid (ii) Higher the pH, weaker the acid (iii) Lower the pH, stronger the base (iv) Lower the pH, weaker the base (a) (i) and (iii) (b) (ii) and (iii) (c) (i) and (iv) (d) (ii) and (iv) Answer: (c) (i) and (iv) Question: 15. The pH of the gastric juices released during digestion is (a) less than 7 (b) more than 7 (c) equal to 7 (d) equal to 0 Answer: (a) less than 7 Question: 16. Which of the following phenomena occur, when a small amount of acid is added to water? (i) Ionisation (ii) Neutralisation (iii) Dilution (iv) Salt formation (a) (i) and (ii) (b) (i) and (iii) (c) (ii) and (iii) (d) (ii) and (iv) Answer: (b) (i) and (iii) Question: 17. Which one of the following can be used as an acid–base indicator by a visually impared student? (a) Litmus (b) Turmeric (c) Vanilla essence (d) Petunia leaves Answer: (c) Vanilla essence Question: 18. Which of the following substance will not give carbon dioxide on treatment with dilute acid? (a) Marble (b) Limestone (c) Baking soda (d) Lime Anwer (d) Lime Question: 19. Which of the following is acidic in nature? (a) Lime juice (b) Human blood (c) Lime water (d) Antacid AnswerAnswer: (b) Human blood Question: 20. In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (Given Figure) was set up. Which among the following statement(s) is(are) correct? (i) Bulb will not glow because electrolyte is not acidic (ii) Bulb will glow because NaOH is a strong base and furnishes ions for conduction. (iii) Bulb will not glow because circuit is incomplete (iv) Bulb will not glow because it depends upon the type of electrolytic solution (a) (i) and (iii) (b) (ii) and (iv) (c) (ii) only (c) (iv) only Answer: (c) (ii) only  MCQs - Part - III (exemplar problem and solution)  Acid Base and Salts Exemplar Problem and Solution (NCERT) MCQs (Multiple Choice Questions - Exemplar Problem and solution) Part - III Question: 21. Which of the following is used for dissolution of gold? (a) Hydrochloric acid (b) Sulphuric acid (c) Nitric acid (d) Aqua regia Answer:- (d) Aqua regia Question: 22. Which of the following is not a mineral acid? (a) Hydrochloric acid (b) Citric acid (c) Sulphuric acid (d) Nitric acid Answer: (b) Citric acid Question: 23. Which among the following is not a base? (a) NaOH (b) KOH (c) NH4OH (d) C2H5 OH Answer: (d) C2H5 OH Question: 24. Which of the following statements is not correct? (a) All metal carbonates react with acid to give a salt, water and carbon dioxide (b) All metal oxides react with water to give salt and acid (c) Some metals react with acids to give salt and hydrogen (d) Some non metal oxides react with water to form an acid Answer: (b) All metal oxides react with water to give salt and acid. Question: 25. Match the chemical substances given in Column (A) with their appropriate application given in Column A Column B (A) Bleaching Powder (i) Preparation of Glass (B) Baking Soda (ii) Production of H2 and Cl2 (C) Washing Soda (iii) Decolourisation (D) Sodium Chloride (iv) Antacid (a) A—(ii), B—(i), C—(iv), D—(iii) (b) A—(iii), B—(ii), C—(iv), D—(i) (c) A—(iii), B—(iv), C—(i), D—(ii) (d) A—(ii), B—(iv), C—(i), D—(iii) Answer: (c) A – (iii), B – (iv), C – (i), D – (ii) Question: 26. Equal volumes of hydrochloric acid and sodium hydroxide solutions of same concentration are mixed and the pH of the resulting solution is checked with a pH paper. What would be the colour obtained? (You may use colour guide given in given Figure (a) Red (b) Yellow (c) Yellowish green (d) Blue Answer: (c) Yellowish green 27. Which of the following is(are) true when HCl (g) is passed through water? (i) It does not ionise in the solution as it is a covalent compound. (ii) It ionises in the solution (iii) It gives both hydrogen and hydroxyl ion in the solution (iv) It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule (a) (i) only (b) (iii) only (c) (ii) and (iv) (d) (iii) and (iv) Answer: (c) (ii) and (iv) 28. Which of the following statements is true for acids? (a) Bitter and change red litmus to blue (b) Sour and change red litmus to blue (c) Sour and change blue litmus to red (d) Bitter and change blue litmus to red Answer: (c) Sour and change blue litmus to red 29. Which of the following are present in a dilute aqueous solution of hydrochloric acid? (a) H3O+ + Cl– (b) H3O+ + OH– (c) Cl– + OH– (d) unionised HCl Answer (a) H3O+ + Cl– 30. Identify the correct representation of reaction occurring during chloralkali process (a) 2NaCl(l) + 2H2O(l) → 2NaOH(l) + Cl2(g) + H2(g) (b) 2NaCl(aq) + 2H2O(aq) → 2NaOH(aq) + Cl2(g) + H2(g) (c) 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(aq) + H2(aq) (d) 2NaCl (aq) + 2H2O (l) → 2NaOH (aq) + Cl2(g) + H2(g) Answer: (d) 2NaCl (aq) + 2H2O (l) → 2NaOH (aq) + Cl2(g) + H2(g)  MCQs - Part - I (exemplar problem and solution)  Acid Base and Salt Exemplar Quesions (NCERT) and Answer Short Answer Type - I Exemplar Problem: 31 - Match the acids given in Column (A) with their correct source given Exemplar Problem: 32. Match the important chemicals given in Column (A) with the chemical formulae given in Column (B) Exemplar Problem: 33. What will be the action of the following substances on litmus paper? Dry HCl gas, Moistened NH3 gas, Lemon juice, Carbonated soft drink, Curd, Soap solution. Answer: Dry HCl – No action Moistened NH3 gas – Turns blue litmus to red Lemon Juice – Turns blue litmus to red Carbonated soft drink – Turns blue litmus to red. Curd – Turns blue litmus to red Soap solution – Turns red litmus to blue. Explanation:- (a) Dry HCl does not liberate hydrogen ion, hence no action takes place with litmus paper. (b) NH3 gas forms ammonium hydroxide in with water which turns red litmus paper blue. (c) Lemon juice is citric acid, so it turns blue litmus paper red. (d) Carbonate soft drink contains carbon dioxide dissolved in water. Carbon dioxide forms carbonic acid; with water; which turns blue litmus paper red. (e) Curd contains lactic acid and hence turns blue litmus paper to red. (f) Soap solution is basic in nature hence it turns red litmus paper to blue. Exemplar Problem:- 34. Name the acid present in ant sting and give its chemical formula. Also give the common method to get relief from the discomfort caused by the ant sting. Answer:- Name of acid present in ant sting: Methanoic acid Chemical Formula of methanoic acid: HCOOH Method to get relief from the discomfort caused by the ant sting: Rubbing baking soda over the location of ant sting. Explanation: Since baking soda is a base thus by rubbing over the ant sting it neutralizes the methanoic acid present in ant sting and gives relief from pain. Exemplar Problem: 35. What happens when nitric acid is added to egg shell? Answer:- Egg shell is made of calcium carbonate. We know that when calcium carbonate reacts with nitric acid, it gives carbon dioxide, calcium nitrate and water. Same reaction happens when nitric acid is poured over egg shell. cid Base and Salt Exemplar Quesions (NCERT) and Answer Short Answer Type - II Exemplar Problem: 36. A student prepared solutions of (i) an acid and (ii) a base in two separate beakers. She forgot to label the solutions and litmus paper is not available in the laboratory. Since both the solutions are colourless, how will she distinguish between the two? Answer: While answering this question, we need to make certain assumptions. Let us assume that laboratory has all the necessary items but litmus paper. We can use phenolphthalein to check which of the beakers contains acid and which one contains a base. Apart from that, we can also use other natural indicators; like China rose or turmeric. Exemplar Problem: 37. How would you distinguish between baking powder and washing soda by heating? Answer:- The production of carbon dioxide is main characteristic of baking soda that makes it suitable for baking. Baking soda gives carbon dioxide and water vapour on heating at even low temperature such as 1000C. The gas so formed turns lime water milky, which confirms the presence of carbon dioxide gas. When Washing soda (Na2CO3) is heated it does not produce carbon dioxide even at high temperature, such as 200⁰C or 300⁰C. However sodium carbonate gives sodium oxide and carbon dioxide when heated at about 1000⁰C. The gas produced by heating of the samples can be checked by passing through lime water. If the lime water turns milky then the evolved gas is carbon dioxide and the heated substance is baking powder. Exemplar Problem: 38: Salt A commonly used in bakery products on heating gets converted into another salt B which itself is used for removal of hardness of water and a gas C is evolved. The gas C when passed through lime water, turns it milky. Identify A, B and C. Answer: Baking power which is a salt is used as bakery products which give sodium carbonate and carbon dioxide gas on heating. Sodium carbonate is used to remove the hardness of water. Carbon dioxide turns lime water milky. Therefore, Salt A, which is sodium bicarbonate and used as baking powder. Salt B is sodium carbonate, which is used to remove the hardness of water. The C is carbon dioxide gas which turns lime water milky. Exemplar Problem: 39: In one of the industrial processes used for manufacture of sodium hydroxide, a gas X is formed as byproduct. The gas X reacts with lime water to give a compound Y which is used as a bleaching agent in chemical industry. Identify X and Y giving the chemical equation of the reactions involved. Answer:- Sodium chloride is used for manufacture of sodium hydroxide which is called Chlor-Alkali Process. In this process chlorine and hydrogen gas are formed as byproducts along with sodium hydroxide. Chlorine gas gives bleaching power when reacts with lime water and used as bleaching agent in chemical industries. Therefore, The gas ‘X’ is chlorine. Compound ‘Y’ is calcium oxychloride which is commonly known as bleaching powder and used as bleaching agent in chemical industries. Exemplar Problem: 40. Fill in the missing data in the following table Exemplar Problem: 41. What are strong and weak acids? In the following list of acids, separate strong acids from weak acids. Hydrochloric acid, citric acid, acetic acid, nitric acid, formic acid, sulphuric acid. Answer:- Strong Acid – Strong acids ionize completely in their aqueous solution and produce maximum number of hydrogen ions. Mineral acids are generally strong acids. Weak Acid – Weak acids do not ionize completely in their aqueous solution. Organic acids are generally weak acids. Strong acid : Hydrochloric acid, Nitric Acid, Sulphuric Acid Weak acid: Citric Acid, Acetic Acid, Formic Acid Exemplar Problem: 42. When zinc metal is treated with a dilute solution of a strong acid, a gas is evolved, which is utilised in the hydrogenation of oil. Name the gas evolved. Write the chemical equation of the reaction involved and also write a test to detect the gas formed. Answer: Zinc metal gives hydrogen gas when it is treated with dilute sulphuric acid. Hydrogen gas is utilized in hydrogenation of oil. Therefore, the gas evolve is hydrogen. Test for hydrogen gas: When a burning candle is brought near the hydrogen gas, it burns with pop sound which confirms the presence of hydrogen gas. Acid Base and Salt Exemplar Quesions (NCERT) and Answer Long Answer Questions - I Exemplar Problem: 43 :- In the following schematic diagram for the preparation of hydrogen gas as shown in Figure 2.3, what would happen if following changes are made? (a) In place of zinc granules, same amount of zinc dust is taken in the test tube (b) Instead of dilute sulphuric acid, dilute hydrochloric acid is taken (c) In place of zinc, copper turnings are taken (d) Sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated. Answer:- (a) When zinc dust is taken instead of zinc granules to react with sulphuric acid, the hydrogen gas is formed. But the rate of reaction increases in the case of zinc dust compared to the zinc granules, because of increase in surface area because increase in surface are increases the rate of reaction. Thus, when zinc dust is used in the place of zinc granules, hydrogen gas is produced at a faster rate. (b) Zinc granules give hydrogen gas; along with zinc chloride; when they react with hydrochloric acid. Thus, when hydrochloric acid is used at the place of sulphuric acid, zinc chloride is formed in place of zinc sulphate; along with hydrogen gas and reaction takes place at the same rate. (c) Copper does not react with dilute acids in normal condition because copper lies at lower position in reactivity series and is considered as a noble metal. Thus, if copper turning is taken in place of zinc granules, no reaction will take place. (d) If sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated, sodium zincate is formed along with hydrogen gas. But heating of test tube will increase the rate of formation of hydrogen gas as heating of reaction mixture increases the rate of reaction. Exemplar Problem: 44 - For making cake, baking powder is taken. If at home your mother uses baking soda instead of baking powder in cake, (a) how will it affect the taste of the cake and why? (b) how can baking soda be converted into baking powder? (c) what is the role of tartaric acid added to baking soda? Answer: (a) The chemical name of baking soda is sodium bicarbonate which gives sodium carbonate, carbon dioxide and water on heating. If baking soda is used instead of baking powder, in making of cake the formation of sodium carbonate would make the taste of cake bitter. (b) By mixing of edible oil, usually tartaric acid, baking soda is converted into baking powder. Other mild edible oil is also used instead of tartaric acid. (c) Tartaric acid gives hydrogen ion when water is added to it. Hydrogen ion produced by tartaric acid reacts with sodium bicarbonate and gives carbon dioxide which makes the dough soft and spongy. If sodium carbonate is formed because of heating, tartaric acid neutralizes it and forms sodium tartarate which has pleasant smell and good taste. Acid Base and Salt Exemplar Quesions (NCERT) and Answer Long Answer Questions - II Exemplar Problem: 45 - A metal carbonate X on reacting with an acid gives a gas which when passed through a solution Y gives the carbonate back. On the other hand, a gas G that is obtained at anode during electrolysis of brine is passed on dry Y, it gives a compound Z, used for disinfecting drinking water. Identity X, Y, G and Z. Answer: Calcium carbonate gives carbon dioxide gas when reacts with hydrochloric acid. Carbon dioxide turns lime water milky when passed through it because of formation of calcium carbonate. When carbon dioxide; so formed; is passed through lime water, lime water turns milky because of the formation of calcium carbonate. On the electrolysis of brine, chlorine gas is deposited over anode which gives calcium oxychloride (Bleaching powder) on passing over slaked lime. The bleaching powder is used in disinfecting the drinking water. Bleaching powder is used for disinfecting drinking water. Therefore, Metal carbonate ‘X’ is calcium carbonate. Solution ‘Y’ is lime water (Calcium hydroxide). Gas ‘G’ is chlorine gas. Dry ‘Y’ is dry calcium hydroxide (dry slaked lime). Compound ‘Z’ is bleaching powder (Calcium oxychloride). Exemplar Problem: 46 - A dry pellet of a common base B, when kept in open absorbs moisture and turns sticky. The compound is also a by–product of chloralkali process. Identify B. What type of reaction occurs when B is treated with an acidic oxide? Write a balanced chemical equation for one such solution. Answer:- Dry pellets of sodium hydroxide absorb moisture and turn sticky when kept in open which is also a byproduct of Chlor-alkali process. Non-metallic oxides are acidic in nature, such as carbon dioxide. When carbon dioxide reacts with sodium hydroxide it gives sodium carbonate and water. Since sodium hydroxide is a base and solution of carbon dioxide is an acid, so reaction taking place between them is a neutralization reaction. Therefore, common base ‘B’ is sodium hydroxide. Reaction between ‘B’ and acidic oxide is neutralization reaction. Exemplar Problem: 47. A sulphate salt of Group 2 element of the Periodic Table is a white, soft substance, which can be moulded into different shapes by making its dough. When this compound is left in open for some time, it becomes a solid mass and cannot be used for moulding purposes. Identify the sulphate salt and why does it show such a behaviour? Give the reaction involved. Answer: Calcium belongs to group 2. Calcium sulphate is a white soft substance. It is known as Plaster of Paris, which can be moulded into different shapes by making its dough. When Plaster of Paris is left for some time in open, it turns into solid mass because of reaction with moisture present in the atmosphere. The solid mass so formed is known as gypsum and cannot be further used for moulding. The above said group 2 element is calcium and sulpahte of that element is Plaster of Paris. Plaster of Paris shows such behavior because of presence half water molecule as water of crystallization. Exemplar Problem: 48. Identify the compound X on the basis of the reactions given below. Also, write the name and chemical formulae of A, B and C. Answer: Therefore, Compound ‘X’ is sodium hydroxide (NaOH). Compound ‘A’ is Zinc sulpahte (ZnSO4). Compound ‘B’ is Sodium chloride (NaCl). Compound ‘C’ is sodium acetate (CH3COONa) Acid Base and Salt Question and Answer In Text Solution(NCERT Book) - Part - I NCERT Book Page Number – 18 Q -1- You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? Answer:- The acidic and basic nature of the solution can be identified by using litmus paper. A basic solution turns red litmus to blue and an acidic solution turns the blue litmus to red. The solution which changes the red litmus to blue is basic solution. Now, between two solutions, which turn the litmus which turned blue because of basic solution, to red, is acidic solution. The last one solution is distilled water. NCERT Book Page Number - 22 Q -1- 1. Why should curd and sour substances not be kept in brass and copper vessels? Answer:- Curd and other sour substances reacts with brass and copper vessels and forms hydrogen and other harmful substances due to presence of acid in them. That’s why curd and other sour substances should not be kept in brass and copper vessels. Q -2 - Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas? Answer:- When acid reacts with metal, it liberates hydrogen gas usually. The hydrogen gas liberates by reaction of acid and metals is tested by using following method – Fit an apparatus as shown in the figure. (Ref:Figure taken from NCERT Book) Take some zinc granules in the test tube and pour sulphuric acid in it. Allow the gas pass through the solution of soap. When gas starts coming out from the soap solution bubbles are formed. Bring a burning candle near the bubbles of gas it burns with a pop sound. Burning of gas with pop sound proves that it is hydrogen gas. Reaction involve in this process:- Q -3- Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride. Answer:- When calcium carbonate reacts with hydrochloric acid, it produces carbon dioxide gas with effervescence. Carbon dioxide used as fire extinguisher. Hence, it extinguishes a burning candle. Therefore, the metal compound A is calcium carbonate. Acid Base and Salt Questions and Answer - Intext (NCERT Book) - Part- II NCERT Page Number – 25 Question: -1:- Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character? Answer: HCl, HNO3, etc. form hydronium ions in aqueous solutions while alcohol and glucose do not dissociates hydronium ion in the solution instead they contain hydrogen. Hence, HCl, HNO3, etc show acidic characters in aqueous solution while solutions of compounds like alcohol and glucose do not. Question: -2:- Why does an aqueous solution of an acid conduct electricity? Answer:- Acid dissociates ions in aqueous solution, which conducts electricity. Question: -3:- Why does dry HCl gas not change the colour of the dry litmus paper? Answer:- Because dry HCl gas does not contain hydronium ions. Hence, it does not changes the colour of dry litmus paper. Question: -4- While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? Answer: When acid is mixed with water it evolves heat. The reaction takes place in this process is exothermic. If water is mixed with acid, then because of presence of large amount of acid, more heat would be evolved, which may splashes out and can be resulted into accident due to burning. Therefore, it is recommended that the acid should be added to water and not water to the acid. Question: -5- How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted? The concentration of hydronium ions (H3O+) decreases when a solution of an acid is diluted. Q -6- How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of sodium hydroxide? Answer: The concentration of hydroxide ions increase when excess base is dissolved in a solution of sodium hydroxide. NCERT Page Number – 28 Q -1- You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic? Answer: An acidic solution has pH value less than 7 and a basic solution has pH value more than 7. Hence, the solution A having pH value is 6 is acidic consequently has more hydrogen ion concentration. The solution B which has pH value is 8 is basic solution. Q -2- What effect does the concentration of H+(aq) ions have on the nature of the solution? Answer: The concentration of H+ (aq) ions determines the acidic nature of solution. Hence, acidity of a solution increases with increases in concentration of H+ (aq) ions and vice versa. Q -3- Do basic solutions also have H+(aq) ions? If yes, then why are these basic? Answer: Yes basic solutions also have H+(aq) ions. They are basic because the concentration of hydroxide (OH-) is more than the H+(aq) ions. Q -4- Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)? Answer: If the soil is acidic then it should be treated with quick lime or slaked lime to make it neutralize so that soil can be made proper for cultivation. NCERT Page Number – 33 Q -1- What is the common name of the compound CaOCl2? Answer: Common name of CaOCl2 is bleaching powder. Q -2- Name the substance which on treatment with chlorine yields bleaching powder. Answer: Calcium hydroxide [Ca(OH)2] yields bleaching powder when treated with chlorine. Q -3- Name the sodium compound which is used for softening hard water. Answer: Sodium carbonate (Washing soda) is used for softening the hard water. Q -4- What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved. Answer: When solution of sodium hydrocarbonate is heated, sodium carbonate, water and carbon dioxide gas is formed. Q -5- Write an equation to show the reaction between Plaster of Paris and water. Answer: Acid Base and Salt NCERT Exercise Solution - Part- I Question: -1- A solution turns red litmus blue, its pH is likely to be (a) 1 (b) 4 (c) 5 (d) 10 Answer - (d) 10 Question: -2:- A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains (a) NaCl (b) HCl (c) LiCl (d) KCl Answer – (b) HCl Question: -3- 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be (a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL Answer – (d) 16 mL Question: -4:- Which one of the following types of medicines is used for treating indigestion? (a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic Answer:- (c) Antacid Question: -5:- Write word equations and then balanced equations for the reaction taking place when – (a) dilute sulphuric acid reacts with zinc granules. (b) dilute hydrochloric acid reacts with magnesium ribbon. (c) dilute sulphuric acid reacts with aluminium powder. (d) dilute hydrochloric acid reacts with iron filings. Answer: (a) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen (b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen (c)Suphuric acid + Aluminium → Aluminium sulphate + Hydrogen (d) Sulphuric acid + Iron → Ferric chloride + Hydrogen Q -6- Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it. Answer:- Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, etc. Fix two nails on a cork, and place the cork in a 100 mL beaker. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in figure. Now pour some glucose solution in the beaker and switch on the current. Repeat with alcohol solution and hydrochloric acid. It is observed that bulb does not glow using glucose or alcohol solution. While if a solution of acid is used instead of glucose or alcohol solution, bulb glows. Bulb glows when the solution of hydrochloric acid is used, because it dissociates hydronium ion in aqueous solution. But glucose and alcohol do not dissociate hydronium ion in the water, which is responsible for the conduction of electricity. Hence solution of glucose and alcohol do not conduct electricity. Q -7- Why does distilled water not conduct electricity, whereas rain water does? Answer: Rain water contains acids and many other substances as impurities, which produce hydronium ion, while distilled water does not contain any ionic substance that can dissociate hydronium ion. That’s why rain water conduct electricity and distilled water does not. Q -8- Why do acids not show acidic behaviour in the absence of water? Answer: Hydronium ions are responsible for behavior as acid. Acids do not dissociate hydronium ion in the absence of water, hence they do not show acidic behavior in absence of water. Acid Base and Salt NCERT Exercise Solution - Part- II Question: -9- Five solutions A,B,C,D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline? Arrange the pH in increasing order of hydrogen-ion concentration. Answer: (a) D - neutral. As its pH value is equal to 7 (b) C – Strongly alkaline. As its pH value is 11. (c) B – Strongly acidic. As its pH value is equal to 1 (d) A – Weakly acidic. As its pH value is equal to 4 and more than solution B. (e) E – Weakly alkaline. As its pH value is equal to 9 and less than C. pH value of the given solution in Increasing order of hydrogen-ion concentration. 11 < 9 < 7 < 4 < 1 Question: -10- Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why? Answer: In test tube A fizzing occur more vigorously. Because HCl is stronger acid than acetic acid. Hence, HCl liberates hydrogen gas more vigorously, which causes fizzing more vigorously. Question: -11- Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer. Answer: The value of pH of milk decreases, when it turns into curd. Curd is acidic in nature. Because an acidic substance has less value of pH. Question: -12- A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd? Answer- (a) By making the milk more alkaline, it is prevent for more time to turn into curd. That’s why milkman shifts the pH of fresh milk to slightly alkaline by adding a very small amount of baking soda to it. (b) As this milk is slightly more alkaline than other, therefore, acid produced to set into curd will be neutralized by baking soda added by milkman. Hence, this milk takes a longer time to set as curd. Q -13- Plaster of Paris should be stored in a moisture-proof container. Explain why? Answer- Because plaster of Paris turns into Gypsum after reacting with moisture present in air. That’s why Plaster of Paris should be stored in a moisture-proof container. Q -14- What is a neutralisation reaction? Give two examples. Answer- When acid is added to base, it produces salt and water after neutralizing each other, such reactions are known as neutralization reaction. Example – 1 When Sodium hydroxide is added to hydrochloric acid. Sodium chloride and water is formed. Example -2- Milk of magnesia, which is a base, is given as antacid in the case of indigestion, to neutralize the more acid produced in stomach. Q -15- Give two important uses of washing soda and baking soda. Answer:- Washing Soda – (a) Washing soda is used to remove the permanent hardness of water. (b) Washing soda is used in making soap. Baking Soda – (a) Baking soda is used as baking powder, in the making of bread, and other many items. (b) Baking soda is used in fire – extinguisher. Acid Base and Salt MCQs Test 1 Question 1: Which of the following is acid? (a) Lemon juice. (b) Soft drink. (c) Soap solution. (d) Lime water.               Question 2:What is the taste of acid? (a) Sour (b) Salty (c) Bitter (d) Sweet               Question 3:Phenolphthalein does show which of the following colour with acid? (a) Red (b) Pink (c) Blue (d) No colour            Question 4:- Phenolphthalein does show which colour when added to a base? (a) Red (b) Pink (c) Blue (d) No colour            Question 5: Blue litmus turns to which colour when it is dipped in base? (a) Red (b) Orange (c) Yellow (d) No change            Question 6: Blue litmus turns to which colour when it is dipped in acid? (a) Red (b) Orange (c) Yellow (d) No change            Question 7: Clove is considered as which of the following type of indicator? (a) pH (b) Synthetic (c) Olfactory (d) Man-made            Question 8: What happens when an acid is mixed with the juice of onion? (a) Onion juice loses its smell (b) No change in the smell (c) Acid turns into base (d) A salt is formed            Question 9: What happens when acid is mixed with a base? (a) Respective salt is formed (b) No change takes place (c) Acid becomes basic (d) Information is insufficient            Question 10: When acid reacts with a metal, which of the following gas is evolved? (a) Hydrogen (b) Oxygen (c) Carbon dixoide (d) None of the above           http://www.excellup.com

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