Find below the NCERT Exemplar problems and their solutions for Class 10 Science Chapter, Acids, Bases and Salts:

Multiple Choice Questions (MCQs)

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) Only (i)

            (b) (i) and (iii)

            (c) (ii) and (iii)

            (d) (i) and (iv)

Answer. (d)

Explanation: The reaction of acid and base is an exothermic reaction and results the formation of salt and water.

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)

Explanation:  Alkaline solutions turn red litmus paper to blue. Excess of acid will reverse the changes so we have to add excess of hydrochloric acid which turns the solution acidic.

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)

Explanation: Calcium chloride is a good dehydrating agent so it is used to absorb moisture from the hydrogen chloride gas.

NCERT Solutions for CBSE Class 10 Science

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)

Explanation:

(a) Blue vitriol =CuSO₄•5H₂O

(b) Baking soda = NaHCO₃

(c) Washing soda=Na₂CO₃•10H₂O

(d) Gypsum = CaSO₄ • 2H₂O

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)

Explanation:  Sodium carbonate (Na₂CO₃) is the salt of carbonic acid (H₂CO₃) and sodium hydroxide (NaOH). Here NaOH is a strong base and carbonic acid is a weak acid.

Question. 6 Calcium phosphate is present in tooth enamel. Its nature is

(a) Basic

(b) Acidic

(c) Neutral

(d) Amphoteric

Answer. (a)

Explanation:  Calcium phosphate Ca₃(PO₄)₂ is salt of weak acid and strong base so it is a basic salt.

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)

Explanation:  pH paper gives greenish blue colour in weak alkaline medium so antacid (Mg(OH)₂) which is an alkaline compound will show greenish blue color on pH paper.

Question. 8 Which of the following gives the correct increasing order of acid 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)

Explanation:  Water is a natural substance, hydrochloric acid is a strong acid as it ionizes completely in water, whereas acetic acid ionizes only partially in water; hence, it is a weak acid.

Question. 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 hydrogen carbonate

(c) After washing with plenty of water applies solution of sodium hydroxide on the hand

(d) Neutralise the acid with a strong alkali

Answer. (b)

Explanation:  Use of sodium hydrogen carbonate neutralizes the effect of acid completely.

Question. 10 Sodium hydrogen carbonate 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. (b)

Explanation:  Reaction of sodium hydrogen carbonate with acetic acid forms sodium acetate and water with carbon dioxide (CO₂) gas.

Question. 11 Common salt besides being used in kitchen can also be used as the raw material for making

(i) Washing soda

(ii) Bleaching powder

(iii) Baking soda

(iv) Slaked lime

            (a) (i) and (ii)

            (b) (i), (ii) and (iv)

            (c) (i), (ii) and (iii)

            (d) (i), (iii) and (iv)

Answer. (c)

Explanation: Common salt (sodium chloride) is used as a raw material for manufacturing of sodium hydroxide, washing soda, bleaching powder, baking soda, hydrochloric acid, chlorine, etc.

Question. 12 One of the constituents of baking powder is sodium hydrogen carbonate, the other constituent is

(a) Hydrochloric acid

(b) Tartaric acid

(c) Acetic acid

(d) Sulphuric acid

Answer. (b)

Explanation: Baking powder is a mixture of sodium hydrogen carbonate and a weak edible acid named as tartaric acid.

Question. 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)

Explanation: The tooth paste commonly used is alkaline or basic in nature. So they can neutralize the effect of extra acids present in mouth cavity which are mainly responsible for tooth decay.

Question. 14 Which of the following statements is correct about an aqueous solution of an acid and 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. (d)

Explanation: The pH values lie between 0 to 14 in which 0 to 7 is acidic medium and 7 to 14 is alkaline or basic medium.

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)

Explanation: Hydrochloric acid is produced by our stomach during digestion.

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) Formation

            (a) (i) and (ii)

            (b) (i) and (iii)

            (c) (ii) and (iii)

            (d) (ii) and (iv)

Answer. (b)

Explanation: Ionization can be defined as the dissociation of molecule into its respective ions.   Mixing of an acid with water decreases the concentration of acid and is called as dilution.

Question. 17 Which one of the following can be used as an acid-base indicator by a visually impaired student?

(a) Litmus

(b) Turmeric

(c) Vanilla essence

(d) Petunia leaves

Answer. (c)

Explanation: An olfactory indicator is required for visually impaired student. Vanilla essence imparts different smell in different medium so can be used for such students.

Question. 18 Which of the following substances will not give carbon dioxide on treatment with dilute acid?

(a) Marble

(b) Limestone

(c) Baking soda

(d) Lime

Answer. (d)

Explanation: Marble or lime stone (CaCO₃) and baking soda (NaHCO₃) release CO₂ with HCl but lime (CaO) with HCl form CaCl₂ and water.

Question. 19 Which of the following is acidic in nature?

(a) Lime juice

(b) Human blood

(c) Lime water

(d) Antacid

Answer. (a)

Explanation: Lime juice contains citric acid so acidic in nature.

Question. 20 In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (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) Only (ii)

            (d) Only (iv)

Answer. (c)

Explanation: Strong acids or bases are good conductors of electricity because they ionize completely in aqueous solution to give ions.

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)

Explanation: Aqua-regia is a very strong acid and can be used to dissolve gold. It is a 1: 3 mixture of concentrated HNO₃ and concentrated HCl.

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)

Explanation: Mineral acids are mainly prepared from the minerals present in the earth’s crust so they are also called as  inorganic acids like HCI (hydrochloric acid), H₂SO₄ (sulphuric acid) and HNO₃ (nitric acid). Organic acids are produced from plants or animals and contain carbon atoms like acetic acid, citric acid.

Question. 23 Which among the following is not a base?

(a) NaOH

(b) KOH

(c) NH₄OH

(d) C₂H₅OH

Answer. (d)

Explanation: C₂H₅OH is an organic compound with –OH functional group that is known as alcohol. It cannot give OH^– ions in its solution.

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)

Explanation: Some of the metal oxides react with water to form alkali like sodium oxide reacts with water to form sodium hydroxide.

Question. 25 Match the chemical substances given in Column I with their appropriate application given in Column II.

Column I	Column II
A. Bleaching Powder	1. Preparation of glass
B. Baking Soda	2. Production of H2 and Cl2
C. Washing Soda	3. Decolourisation
D. Sodium Chloride	4. Antacid

Codes:

            A         B         C         D

(a)       2          1          4          3

(b)       3          2          4          1

(c)       3          4          1          2

(d)       2          4          1          3

Answer. (c)

Explanation:

• Bleaching powder (CaOCl₂) – Used for bleaching or decolourisation of clothes.
• Baking soda (NaHCO₃)- Used as an antacid for relieving stomach acidity.
• Washing soda(Na₂CO₃)- Used in preparation of glass.
• Sodium chloride (NaCl)- Used for production of H₂ and Cl₂ gases.

Short Answer Type Questions:

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 in the figure given below).

(a) Red

(b) Yellow

(c) Yellowish green

(d) Blue

Answer. (c)

Explanation: Hydrochloric acid and sodium hydroxide are strong acid and base respectively. So mixing of strong acid and strong base will form neutral salt and water giving the pH value as 7.

Question. 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 o hydrogen ion with water molecule.

            (a) Only (i)

            (b) Only (iii)

            (c) (ii) and (iv)

            (d) (iii) and (iv)

Answer. (c)

Explanation: HCI being a polar covalent compound which can easily ionize in water to form hydronium ions (H₃O⁺) and chloride ions (Cl^–).

Question. 28 Which of the following statement 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)

Explanation: Acids have sour taste and can turn blue litmus solution to red.

Question. 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)

Explanation: A strong acid like HCl dissociates completely in its aqueous solution and form hydronium ions with its respective anion.

HCl  +  H₂O  →  H⁺  +  Cl⁻  +  H₂O

H₂O  +  H⁺   →  H₃O⁺ (Hydronium ion)

Question. 30 Identify the correct representation of reaction occurring during chloralkali process.

(a) 2NaCl (l)  +  2H₂O (l)  →  2NaOH (l)  +  Cl₂ (g)  +  H₂ (g)

(b) 2NaCl (l)  +  2H₂O (aq)  →  2NaOH (aq)  +  Cl₂ (g)  +  H₂ (aq)

(c) 2NaCl (aq)  +  2H₂O (l)  →  2NaOH (aq)  +  Cl₂ (aq)  +  H₂ (aq)

(d) 2NaCl (aq)  +  2H₂O (l)  →  2NaOH (aq)  +  Cl₂ (g)  +  H₂ (g)

Answer. (d)

Explanation: Chlor-alkali process is an industrial process that is used to form sodium hydroxide by electrolysis of aqueous solution of sodium chloride. It forms sodium hydroxide with hydrogen and chlorine gas as by-products.

Short Answer Type Questions

Question. 31 Match the acids given in Column I with their correct source given in Column II

Column I	Column II
A. Lactic Acid	1. Tomato
B. Acetic Acid	2. Lemon
C. Citric Acid	3. Vinegar
D. Oxalic Acid	4. Curd

 Answer.

Column I	Column II
A. Lactic Acid	4. Curd
B. Acetic Acid	3. Vinegar
C. Citric Acid	2. Lemon
D. Oxalic Acid	1. Tomato

Question. 32 Match the important chemicals given in Column I with the chemical formulae given in Column II.

Column I	Column II
A. Plaster of Paris	1. Ca(OH)2
B. Gypsum	2. CaSO4.1/2H2O
C. Bleaching Powder	3. CaSO4.2H2O
D. Slaked Lime	4. CaOCl2

Answer.

Column I	Column II
A. Plaster of Paris	2. CaSO4.1/2H2O
B. Gypsum	3. CaSO4.2H2O
C. Bleaching Powder	4. CaOCl2
D. Slaked Lime	1. Ca(OH)2

Question. 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 HCI gas: It will show no effect on litmus paper as no ionization will occur in gaseous state.
• Moistened NH₃ gas: It is alkaline in nature so red litmus will turn blue.
• Lemon Juice- it contains citric acid therefore it turns blue litmus to red.
• Carbonated soft drinks: They have carbonic acid which is a weak acid and can turns blue litmus to red.
• Curd: It contains lactic acid so blue litmus will turn red.
• Soap solution: They are alkaline in nature and can turn red litmus to blue.

Question. 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.

Ans. Acid present in ant sting is formic acid which is also named as Methanoic acid.

Chemical formula for formic acid is HCOOH.

It is acidic in nature, so an alkaline compound like wet baking soda can be used to neutralize its effect to get relief from discomfort.

Question. 35 What happens when nitric acid is added to egg shell?

Answer.

Calcium carbonate is main component of egg shell so when we add nitric acid to egg shell, it reacts with calcium carbonate to form soluble calcium nitrate and water with brisk effervescence of carbon dioxide gas.

CaCO₃ (s)  +  2HNO₃ (aq)  →  Ca(NO₃)₂ (aq)  +  CO₂ (g)  +  H₂O (l)

Question. 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.

Other then litmus paper, pH indicators can also be used to distinguish between an acidic and an alkaline solution. Some common pH indicators are methyl orange, phenolphthalein. They show characteristic colour change in acidic and alkaline medium. Some natural indicators such as turmeric, red cabbage juice can also used to identify acidic and alkaline behavior of solutions.

pH Indicator	Colour in acidic medium	Colour in basic medium
Phenol red	Yellow	Red
Phenolphthalein	Colourless	Pink
Methyl Orange	Red/Pink	Yellow
Turmeric	Yellow	Reddish Brown
Red cabbage juice	Red	Yellow

Question. 37 How would you distinguish between baking powder and washing soda by heating?

Answer.

Baking soda is sodium hydrogen carbonate (NaHCO₃) which forms sodium carbonate (Na₂CO₃), water and carbon dioxide gas while heating.

2 NaHCO₃  →  Na₂CO₃  +  H₂O  +  CO₂↑

Presence of CO₂ can be detected with the help of lime water which turns milky after reacting with carbon dioxide gas

Washing soda is sodium carbonate deca-hydrate (Na₂CO₃.10H₂O) which means there are 10 water molecules present with crystal hence named as water of crystallization.

On heating washing soda, it becomes anhydrous and presence of water of crystallization can be detected with the help of anhydrous CuSO₄ which shows colour change from white to blue with moisture.

Na₂CO₃.10H₂O  →  Na₂CO₃  +  10 H₂O

Question. 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.

Salt which is used in bakery product is baking soda or sodium hydrogen carbonate. On heating of baking soda, sodium carbonate is formed with evolution of carbon dioxide gas.

2 NaHCO₃  →  Na₂CO₃  +  H₂O  +  CO₂↑

Sodium carbonate is used to remove hardness of water. Another product is carbon dioxide gas that gives lime test in which it turns lime water milky.

Thus, A = NaHCO₃; B= Na₂CO₃ and C= CO₂.

Question. 39 In one of the industrial processes for manufacture of sodium hydroxide, a gas X is formed as by-product. 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 hydroxide is manufactured by electrolysis of aqueous solution of sodium chloride which forms hydrogen and chlorine gas as byproducts. Reaction of chlorine gas with lime water forms bleaching powder (CaOCl₂). Thus, X = Cl₂ gas and Y = Calcium oxychloride (CaOCl₂)

The equation for the preparation of sodium hydroxide (NaOH) and bleaching powder(CaOCl₂), is given below:

(i) 2NaCl (aq) + 2H₂O (l) →  2NaOH (aq) + Cl₂ (g) + H₂ (g)

(ii) Cl₂ (g) + Ca(OH)₂ →  CaOCl₂  +  H₂O

Question. 40 Fill in the missing data in the given table.

Name of the salt	Formula	Salt obtained from
Base	Acid
Ammonium Chloride	NH4Cl	NH4OH	–
Copper Sulphate	–	–	H2SO4
Sodium Chloride	NaCl	NaOH	–
Magnesium Nitrate	Mg(NO3)2	–	HNO3
Potassium Sulphate	K2SO4	–	–
Calcium Nitrate	Ca(NO3)2	Ca(OH)2	–

Answer.

Name of the salt	Formula	Salt Obtained from
Base	Acid
Ammonium Chloride	NH4Cl	NH4OH	HCl
Copper Sulphate	CuSO4	Cu(OH)2	H2SO4
Sodium Chloride	NaCl	NaOH	HCl
Magnesium Nitrate	Mg(NO3)2	Mg(OH)2	HNO3
Potassium Sulphate	K2SO4	KOH	H2SO4
Calcium Nitrate	Ca(NO3)2	Ca(OH)2	HNO3

Question. 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 acids ionize completely in its aqueous solution therefore such acids contains high concentration of hydronium ions in their solutions. Nitric acid, sulphuric acid are good examples of strong acids.

Unlike strong acids, weak acids cannot ionize completely therefore solution of a weak acid contains some un-ionized molecules with its ions. Organic acids like citric acid, acetic acid are weak acids.

In the given acids;

• Hydrochloric acid – Strong Acid
• Citric acid- Weak Acid
• Acetic acid- Weak Acid
• Nitric acid- Strong Acid
• Formic acid- Weak Acid
• Sulphuric acid- Strong Acid

Question. 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 is a metallic element. Metals react with dilute solutions of strong acids to form respective salts and hydrogen gas.

The reaction of zinc with dilute solution of strong acid like HCl forms zinc chloride (ZnCl₂) and hydrogen gas is evolved.

Zn  +  2HCl  →  ZnCl₂  +  H₂↑

Hydrogen gas is used in hydrogenation of vegetable oil. It can be tested with the help of burning splinter as it burns with a pop sound.

Long Answer Type Questions

Question. 43 In the following schematic diagram for the preparation of hydrogen gas as shown in the figure, what would happen if the 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) Since zinc dust has larger surface area than zinc granules, so reaction will be faster and hydrogen gas will evolve with greater speed.

(b) Both hydrochloric acid and sulphuric acid are strong acids so almost same amount of gas is evolved.

(c) Copper is a less reactive metal and does not react with dilute acids so there will be no reaction between copper turnings and dilute HCl.

(d) Reaction of zinc with sodium hydroxide forms sodium zincate (Na2ZnO2) with evolution of hydrogen gas.

Zn  +  2NaOH  →  Na₂ZnO₂  +  H₂↑

Question. 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) Baking soda is sodium hydrogen carbonate which decomposes to sodium carbonate, water and carbon dioxide on heating. Baking powder is a mixture of sodium hydrogen carbonate with tartaric acid which readily reacts with sodium carbonate and neutralizes it. Therefore use of baking soda will give a bitter taste to cake due to presence of sodium carbonate.

(b) Baking powder is formed by addition of tartaric acid to baking soda.

(c) Presence of tartaric acid in baking powder neutralizes the effect of sodium carbonate formed during decomposition of baking soda.

Question. 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. Identify X, Y, G and Z.

Answer.

Here X is calcium carbonate which is a metal carbonate which reacts with dilute HCl to form calcium carbonate (CaCO₃), water and carbon dioxide gas.

CaCO₃ + dil. 2HCl →  CaCl₂ + H₂O + CO₂↑

When carbon dioxide gas is passed through lime water or calcium hydroxide (Ca(OH)₂), it gives the calcium carbonate back. So Y is calcium hydroxide.

Ca(OH)₂  +  CO₂  →  CaCO₃↓  +  H₂O

Brine is saturated solution of sodium chloride (NaCl). Electrolysis of brine forms hydrogen gas at cathode  and chlorine gas at anode.

So G is chlorine gas which is passed through dry calcium hydroxide (Ca(OH)₂), produces bleaching powder, CaOCl₂ (Z).

Ca(OH)₂  +  Cl₂ →  CaOCl₂  +H₂O

Hence, X= Calcium carbonate; Y= Lime water; G= Chlorine gas and Z =Calcium oxychloride or bleaching powder

Question. 46 A dry pellet of a common base B, when kept in open absorbs moisture and turns sticky. The compound is also a product of chlor-alkali 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.

Here common base B is sodium hydroxide which is hygroscopic and absorbs moisture from the atmosphere. It is also formed as a product of chlor-alkali process. Chlor-alkali process is electrolysis of brine (saturated solution of sodium hydroxide) that forms aqueous sodium hydroxide with hydrogen gas and chlorine gas.

Reaction of sodium hydroxide with acidic oxide like carbon dioxide forms salt (sodium carbonate) and water.

2NaOH  +  CO₂  →  Na₂CO₃  +  H₂O

Question. 47 A sulphate salt of group-2 element of the periodic table is a white, soft substance, which can be molded 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 molding purposes. Identify the sulphate salt and why does it show such a behaviour? Give the reaction involved.

Answer.

The sulphate salt whichcan be molded into different shapes by making its dough is calcium sulphate hemi-hydrate (CaSO₄.1/2H₂O). It is commonly known as plaster of Paris. Here, two molecules of calcium sulphate share one molecule of water.

Because of presence of water crystallization, it is soft.

It readily reacts with atmospheric moisture and forms hard solid mass. This solid mass is known as gypsum (CaSO₄.2H₂O). Conversion of plaster of Paris to gypsum can be represented as given below:

CaSO₄.1/2H₂O  +  ½ H₂O  →  CaSO₄.2H₂O

Question. 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.

Ans. X must be a compound which forms water with acids. It means it must be a base which reacts with acids to form salt and water. This base also reacts with zinc metal and releases hydrogen gas. So it must be NaOH (sodium hydroxide).