2. Acids, Bases and Salts Class 10 NCERT Exemplar Solutions (CBSE 2026 Guide)

Class 10 Science Chapter 2: Acids, Bases and Salts : NCERT Exemplar Solutions with Detailed Explanation

Chapter 2: Acids, Bases and Salts

Multiple Choice Questions

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)

[ When a solution of an acid is mixed with a solution of a base, a chemical reaction called neutralization occurs. In this reaction, the acid and base react to form water and a salt. The temperature change during the reaction depends on the specific acid and base involved, but generally, the reaction is exothermic, meaning it releases heat.]

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 (HCl) .

[ An aqueous solution turning red litmus solution blue indicates that the solution is basic. To reverse this change and bring the solution back to acidic conditions, you would need to add an acidic solution.]

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 ions from the evolved gas

Answer : (c) absorb moisture from the gas

[ The role of calcium chloride in the guard tube is to absorb moisture from the hydrogen chloride gas. Calcium chloride is a hygroscopic substance, meaning it has a strong affinity for water and can absorb moisture from the surrounding air or gases. In this case, it helps to dry the hydrogen chloride gas by removing any water vapor present in it during its preparation on a humid day. This ensures that the hydrogen chloride gas collected is dry and free from moisture.]

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.

[ Baking soda (sodium bicarbonate, NaHCO₃) does not contain water of crystallization. It is a hydrate, but the water is not present in a fixed ratio as water of crystallization. Blue vitriol (copper sulfate pentahydrate, CuSO₄ . 5H₂O), washing soda (sodium carbonate decahydrate, Na₂CO₃ .10H₂O), and gypsum (calcium sulfate dihydrate, CaSO₄ . 2H₂O) all contain water of crystallization in their structures.]

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

[ Sodium carbonate (Na₂CO₃) is a basic salt because it is a salt of a weak acid (carbonic acid, H₂CO₃) and a strong base (sodium hydroxide, NaOH). ]

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

(a) basic (b) acidic (c) neutral (d) amphoteric

Answer : (a) basic

[ In the case of tooth enamel, calcium phosphate tends to act as a basic substance, helping to neutralize acids and maintain the pH balance in the mouth.]

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

[ Antacids are substances that are often basic and are used to neutralize excess stomach acid. Therefore, adding an antacid to the solution would likely change the color of the pH paper to greenish-blue.]

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

[ In this order, water is neutral, hydrochloric acid is a strong acid, and acetic acid is a weak 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.

[ In case of a spill of concentrated acid on the skin, it is important to immediately wash the affected area with plenty of water to dilute and remove the acid. The use of sodium hydrogen carbonate (baking soda) in the paste can help neutralize any remaining acid. Sodium hydrogen carbonate is a mild base that can react with the acid, forming water and a salt. This can be an effective first aid measure in dealing with acid spills on the skin.]

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)

[ When sodium hydrogencarbonate (baking soda) is added to acetic acid, it undergoes a chemical reaction, producing carbon dioxide gas. Let's analyze the statements:

(i) It turns lime water milky - True. The evolved carbon dioxide reacts with lime water (calcium hydroxide) to form calcium carbonate, which is insoluble and gives a milky appearance.

(ii) It extinguishes a burning splinter - True. Carbon dioxide is not combustible and can extinguish a burning splinter by displacing oxygen.

(iii) It dissolves in a solution of sodium hydroxide - False. Carbon dioxide does not dissolve in a solution of sodium hydroxide.

(iv) It has a pungent odor - False. Carbon dioxide is colorless and odorless.]

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) and (iii) (d) (i), (iii) and (iv)

Answer : (a) (i) and (ii) .

[ Common salt (sodium chloride, NaCl) can be used as a raw material for making:

(i) washing soda (sodium carbonate, Na₂CO₃) (ii) bleaching powder (calcium oxychloride, Ca(OCl)₂) ]

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

[ Tartaric acid is a common acid used in baking powder to react with sodium hydrogencarbonate and produce carbon dioxide gas, contributing to the leavening process in baking.]

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

[ Toothpaste commonly used is slightly basic or alkaline in nature. The alkalinity helps in neutralizing acids produced by bacteria in the mouth, which can contribute to tooth decay.]

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 : (d) (ii) and (iv)

[ (ii) Higher the pH, weaker the acid. (iv) Lower the pH, weaker the base.

The pH scale is a measure of the acidity or basicity of a solution. Lower pH values indicate stronger acids, while higher pH values indicate weaker acids. On the other hand, higher pH values indicate stronger bases, while lower pH values indicate weaker bases.]

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

[ The pH of gastric juices released during digestion is generally less than 7 .The gastric juice in the stomach is acidic, and its pH is typically around 1.5 to 3.5. ]

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)

[ (i) Ionisation – When an acid is added to water, it dissociates to release H⁺ ions (or H₃O⁺ in water). So yes, ionisation occurs.

(ii) Neutralisation – This requires an acid and a base to react. Adding acid alone to water does not involve a base, so neutralisation does not occur.

(iii) Dilution - The acid is diluted as it mixes with water, resulting in a less concentrated solution.

(iv) Salt formation – This requires both an acid and a base (or metal, etc.) to react. Simply mixing acid with water does not form a salt.

So the phenomena occurring: (i) and (iii). ]

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

[ Vanilla essence can be used as an acid-base indicator by a visually impaired student. Vanilla essence is sensitive to pH changes and can exhibit different smells under acidic and basic conditions. It has a sweet aroma in a basic (alkaline) solution and a slightly bitter aroma in an acidic solution.]

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

Answer : (d) Lime

[ Marble and limestone are both forms of calcium carbonate (), and they will release carbon dioxide when treated with dilute acid.

Baking soda (sodium bicarbonate, ) will also release carbon dioxide when treated with an acid and lime (calcium oxide, CaO) will not produce carbon dioxide when treated with dilute acid because it is already an oxide and does not contain carbonate ions.]

19. Which of the following is acidic in nature?

(a) Lime juice (b) Human blood (c) Lime water (d) Antacid

Answer : (a) Lime juice

[ Lime juice is acidic in nature. It contains citric acid, which imparts a sour taste to the juice. The other options, human blood, lime water, and antacid, are not acidic ]

20. In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (Figure 2.1) was set up. Which among the following statement(s) is(are) correct?

Fig. 2.1

(i) Bulb will not glow because the 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 the 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 (d) (iv) only

Answer : (c) (ii) only

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

[ Aqua regia, a mixture of concentrated hydrochloric acid and concentrated nitric acid, is used for dissolving gold.]

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

[ Citric acid is not a mineral acid. It is an organic acid found naturally in citrus fruits like lemons and oranges. On the other hand, hydrochloric acid, sulfuric acid, and nitric acid are mineral acids, which are inorganic acids commonly prepared from minerals or other inorganic compounds.]

23. Which among the following is not a base?

(a) NaOH (b) KOH (c) (d)

Answer : (d)

[ , also known as ethanol, is not a base. It is an alcohol. The other options, NaOH (sodium hydroxide), KOH (potassium hydroxide), and (ammonium hydroxide), are all bases.]

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

[ Metal oxides typically react with water to form metal hydroxides (alkaline solutions) and not salts and acids.]

25. Match the chemical substances given in Column (A) with their appropriate application given in Column (B)

Column (A)	Column (B)
(A) Bleaching powder	(i) Preparation of glass
(B) Baking soda	(ii) Production of and
(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)

[ Explanation: (A) Bleaching powder is used for decolourisation (iii).

(B) Baking soda is used as an antacid (iv).

(D) Sodium chloride is not directly matched in the given options, but its common uses include seasoning in food. So, the correct option is (c).

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 Figure 2.2

(a) Red (b) Yellow (c) Yellowish green (d) Blue

Answer : (c) Yellowish green.

[ The mixing of equal volumes of hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions of the same concentration results in a neutralization reaction, forming water and a salt. The pH of the resulting solution should be close to 7, making it neutral. Therefore, the color obtained with pH paper would likely be: (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)

[ (ii) It ionises in the solution (iv) It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule

When hydrogen chloride (HCl) gas is passed through water, it dissolves and undergoes ionization in the solution. The ionization leads to the formation of hydronium ions (H₃O⁺), which is a combination of hydrogen ions (H⁺) and water molecules. ]

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

[ The correct statement for acids is that they are sour in taste and change the color of blue litmus paper to red.]

29. Which of the following are present in a dilute aqueous solution of hydrochloric acid?

(a) (b) (c) (d) unionised HCl

Answer : (a)

[ In a dilute aqueous solution of hydrochloric acid (HCl), the predominant species are ions formed due to the dissociation of the acid. Hydrochloric acid dissociates into hydrogen ions() and chloride ions ()in water.]

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

(a)

(b)

(c)

(d)

Answer : (d)

[ This represents the electrolysis of sodium chloride (NaCl) in aqueous solution, resulting in the formation of sodium hydroxide (NaOH), chlorine gas (), and hydrogen gas ().]

Short Answer Questions

31. Match the acids given in Column (A) with their correct source given in Column (B)

Column (A)	Column (B)
(a) Lactic acid	(i) Tomato
(b) Acetic acid	(ii) Lemon
(c) Citric acid	(iii) Vinegar
(d) Oxalic acid	(iv) Curd

Answer: The correct matching of acids with their sources is:

(a) Lactic acid → (iv) Curd

(b) Acetic acid → (iii) Vinegar

(d) Oxalic acid → (i) Tomato

So, the correct matching is: (a) (iv), (b) (iii), (c) (ii), (d) (i)

32. Match the important chemicals given in Column (A) with the chemical formulae given in Column (B)

Column (A)	Column (B)
(a) Plaster of Paris	(i)
(b) Gypsum	(ii)
(c) Bleaching Powder	(iii)
(d) Slaked Lime	(iv)

Answer : The correct matching of the chemicals with their chemical formulae is:

(a) Plaster of Paris → (ii) CaSO₄·1/2 H₂O

(b) Gypsum → (iii) CaSO₄·2H₂O

(d) Slaked Lime → (i) Ca(OH)₂

So, the correct matching is: (a) (ii), (b) (iii), (c) (iv), (d) (i)

33. What will be the action of the following substances on litmus paper?

Dry HCl gas, Moistened gas, Lemon juice, Carbonated soft drink, Curd, Soap solution.

Answer: The action of the following substances on litmus paper is given in the table below:

Substance	Nature (Acidic/Basic)	Effect on Litmus Paper
Dry HCl gas	Acidic (but no effect in dry state)	No change in dry litmus paper
Moistened NH₃ gas	Basic	Turns red litmus blue
Lemon juice	Acidic	Turns blue litmus red
Carbonated soft drink	Acidic	Turns blue litmus red
Curd	Acidic	Turns blue litmus red
Soap solution	Basic	Turns red litmus blue

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 : The acid present in an ant sting is methanoic acid (also known as formic acid). Its chemical formula is HCOOH.

To get relief from the discomfort caused by an ant sting, a common method is to rub a mild base like baking soda (sodium hydrogencarbonate) over the affected area. This neutralises the effect of the acid and gives relief.

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

Answer : Eggshell contains Calcium carbonate. When Nitric acid is added, brisk effervescence occurs due to release of CO₂ gas.

Reaction: CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂

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 : She can use natural indicators like Turmeric or China rose indicator.

Turmeric turns reddish-brown in a base but stays yellow in an acid. China rose turns pink in an acid and green in a base.

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

Answer : To distinguish between baking powder and washing soda by heating:

Substance	Effect on Heating	Observation
Baking powder (Sodium hydrogencarbonate, NaHCO₃)	Decomposes to produce carbon dioxide gas	Brisk effervescence (bubbles) due to CO₂ gas, which turns lime water milky.
Washing soda (Sodium carbonate decahydrate, Na₂CO₃·10H₂O)	Loses water of crystallisation	On heating, it first loses water molecules and becomes anhydrous white powder (no CO₂ gas is released). No effervescence occurs.

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 A commonly used in bakery products is sodium bicarbonate (). On heating, it gets converted into another salt B, sodium carbonate (), and a gas C, carbon dioxide (), is evolved.

When gas C (carbon dioxide) is passed through lime water (calcium hydroxide,), it turns milky due to the formation of calcium carbonate ().

39. In one of the industrial processes used 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 : In the manufacture of sodium hydroxide (chlor-alkali process), the gas X formed is Chlorine.

It reacts with lime water to form Y, Bleaching powder, used as a bleaching agent.

Reactions: 2NaCl + 2H₂O → 2NaOH + H₂ + Cl₂
and Cl₂ + Ca(OH)₂ → CaOCl₂ + H₂O

40. Fill in the missing data in the following table :

Name of the salt

Formula

Salt obtained from

Base

Acid

(i) Ammonium

(ii) Copper sulphate

(iii) Sodium Chloide

(iv) Magnesium nitrate

(v) Potassium sulphate

(vi) Calcium nitrate

………

NaCl

……….

NaOH

……….

…………

…………..

…………

Answer: The table is completed as follows:

Name of the salt	Formula	Salt obtained from
Name of the salt	Formula	Base	Acid
(i) Ammonium chloride	NH₄Cl	NH₄OH	HCl
(ii) Copper sulphate	CuSO₄	Cu(OH)₂	H₂SO₄
(iii) Sodium chloride	NaCl	NaOH	HCl
(iv) Magnesium nitrate	Mg(NO₃)₂	Mg(OH)₂	HNO₃
(v) Potassium sulphate	K₂SO₄	KOH	H₂SO₄
(vi) Calcium nitrate	Ca(NO₃)₂	Ca(OH)₂	HNO₃

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 are acids that completely ionise in aqueous solution, producing a large number of H⁺ ions.

Weak acids are acids that partially ionise in aqueous solution, producing fewer H⁺ ions.

Strong acids: Hydrochloric acid, nitric acid and sulphuric acid.

Weak acids: Citric acid, acetic acid and formic acid.

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 : The gas evolved is Hydrogen, which is used in hydrogenation of oils.

Reaction: Zn + 2HCl → ZnCl₂ + H₂

Test for the gas: Bring a burning matchstick near it; hydrogen burns with a ‘pop’ sound, confirming its presence.

Long Answer Questions

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) The reaction will still occur, producing hydrogen gas. The rate of reaction might increase due to the increased surface area of zinc particles.

(b) The reaction will still occur, producing hydrogen gas.

(c) Copper does not react with dilute acids to produce hydrogen gas under normal conditions. There will be no evolution of hydrogen gas.

(d) The correct equation for the reaction with sodium hydroxide is:

This reaction involves the formation of sodium zincate and the evolution of hydrogen gas.

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) If only Baking soda is used, the cake may taste bitter due to formation of Sodium carbonate on heating.

(b) Baking soda can be converted into baking powder by mixing it with a mild edible acid like Tartaric acid.

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 : X = Calcium carbonate ; Y = Calcium hydroxide (lime water) ; G = Chlorine ; Z = Bleaching powder

Reactions: (i) CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
(ii) CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

(iii) Cl₂ + Ca(OH)₂ → CaOCl₂ + H₂O

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: B is Sodium hydroxide. It is hygroscopic and becomes sticky by absorbing moisture; it is also formed in the chlor-alkali process. When B reacts with an acidic oxide, a neutralisation reaction occurs.

Example reaction: 2NaOH + CO₂ → Na₂CO₃ + H₂O

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 : The sulphate salt described is calcium sulphate hemihydrate (), commonly known as Plaster of Paris.

Plaster of Paris is a white, soft substance that can be moulded into different shapes when mixed with water to form a dough. When Plaster of Paris is left in open air, it reacts with moisture from the atmosphere and converts into a hard, solid mass, losing its moulding properties.

The reaction involved when Plaster of Paris sets into a hard solid mass is as follows:

Calcium sulphate hemihydrate (Plaster of Paris) reacts with water to form calcium sulphate dihydrate (gypsum), which is a solid mass.

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 : (i) Compound X + Zn → A + Hydrogen gas.

A – Sodium zincate () .

(ii) Compound X + HCl → B + Water.

B – Sodium Chloride (NaCl)

Compound X (NaOH) reacts with HCl to form B (NaCl) and water.

(iii) Compound X Water

C – Sodium acetate ()