NCERT solutions Acids Bases and Salts

NCERT Questions with solutions- Acids, Bases and salts

This article contains NCERT in-text questions and textbook exercise given at the end of the chapter Acids, Bases and salts.

NCERT in-text questions

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?

Ans. A) First put red litmus paper in all three test tubes, one by one. The solution which turns red litmus to blue will be basic solution.

B) Now, add blue litmus paper (formed earlier) to the remaining two test tubes one by one. The solution which converts blue litmus paper red is acidic solution.

C) Solution having no effect on red litmuspaper as well as on blue litmus paper is neutral and thus, it is distilledwater.

2. Why should curd and sour substances not be kept in brass and copper vessels?

Ans. Curd and sour substances have acidic nature, while brass and copper are metals. Acids react with metals to produce hydrogen gas which causes corrosion in metals.

3. 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?

Ans. Generally, hydrogen gas is liberated when an acid reacts with a metal.

For example, when iron reacts with sulphuric acid, hydrogen gas is evolved and salt ferrous sulphate is produced. The reaction is given below-

Fe (s)+ H₂SO₄ (aq)   ⟶  FeSO₄ (aq) + H₂(g)

It can be tested by bringing a burning candle close to the gas filled inside the soap bubbles. Hydrogen gas is highly combustible and burns with a pop sound.

4. A 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.

Ans. The gas evolved extinguishes a burning candle therefore it must be carbon dioxide (CO₂). And it is given the other product is calcium chloride. Now, if we try to rewrite the chemical reaction and fill the missing reactant which will be calcium carbonate (CaCO₃). And water is also produced as a product.

Balanced chemical reaction is given below-

CaCO₃ + 2HCI (aq)  ⟶    CaCl₂ (aq) + CO₂ (g)+ H₂O

5. Why do HCl, HNO₃, etc. show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Ans. H⁺ (aq) or H₃O⁺ ions are produced by HCL and HNO₃ respectively in aqueous solution and therefore HCL and HNO₃ are acidic.

HCl + H₂O ⟶    H₃O⁺ +   Cl^-

HNO₃ + H₂O ⟶   H₃O⁺ + NO₃ ^-

While glucose and alcohol do not produce H⁺ (aq) or H₃O⁺ ions and therefore do not show acidic character.

6. Why does an aqueous solution of acid conduct electricity?

Ans. An aqueous solution of an acid conducts electricity because it produces ions. The electric current is carried through the solution by ions.

7. Why does dry HCl gas not change the color of the dry litmus paper?

Answer. Dry HCl gas cannot show acidic character and therefore, does not change the colour of the dry litmus paper.

8. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans. The dissolution of an acid in water produces a lot of heat as it is a highly exothermic reaction. If water is added to concentrated acid, the heat generated may cause the mixture to splash out and cause burns. Therefore, acid must be added slowly to water with constant stirring.

9. How is the concentration of hydronium ions(H₃O⁺) affected when a solution of an acid is diluted?

Ans. On the dilution of acid, the concentration of hydronium ion per unit volume decreases.

10. How is the concentration of hydroxide ions (OH^-) affected when excess base is dissolved in a solution of sodium hydroxide?

Ans. Addition of excess base to a solution of sodium hydroxide causes increase in the concentration of hydroxyl ions (OH^-).

11. 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?

Ans. Solution A, pH =6 has more concentration of H⁺ ions, so, solution A is acidic and solution B, pH =8 is basic in nature. 

12. What effect does the concentration of H⁺ ions have on the nature of the solution?

Ans.  Increase in concentration of hydrogen (aq) ions increases acidic character and decrease concentration of H⁺ (aq) ions decreases acidic character of the solution.

13. Do basic solutions also have H⁺ (aq) ions? If yes, then why are these basic?

Ans.  Yes, basic solutions also have H⁺ (aq) ions. All aqueous solutions have H⁺ (aq) ions as well as OH^- ions. But the concentration of OH ions is higher than that of H⁺ (aq) ions and hence the solution is basic.

14. 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)?

Ans. If the soil condition is more acidic than optimum conditions than the farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)

15. What is the common name of the compound CaOCl₂?

Ans. Bleaching powder.

16. Name the substance which on treatment with chlorine yields bleaching powder.

Ans. Dry slaked lime, Ca(OH)₂ on treatment with chlorine yields bleaching powder.

17. Name the sodium compound which is used for softening hard water.

Ans. Sodium carbonate (washing soda) is used for softening hard water.

18. What will happen if a solution of sodium hydrogen carbonate is heated? Give the equation of the reaction involved.

Ans.  If a solution of sodium hydrogen carbonate is heated, it gives sodium carbonate, carbon dioxide and water.

2NaHCO₃   +   Heat ⟶       Na₂CO₃+ H₂O +CO₂

19. Write an equation to show the reaction between Plaster of Paris and water.

Ans.  The reaction between Plaster of Paris and water forms gypsum giving a hard solid mass.

CaSO₄.1/2H2O + 1.5H₂O  ⟶     CaSO₄.2H₂O

Plaster of Paris                                 Gypsum

Textbook exercise questions

1. A solution turns red litmus blue; its pH is likely to be

(a) 1

b) 4

(c) 5

(d) 10

Ans. (d)

2. A solution reacts with crushed egg-shells to give a gas that turns lime water milky.

The solution contains

(a) NaCl

(b) HC1

(c) LiCl

(d) KC1.

Ans. b)

3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HC1. If we take 20 mL of the same solution of NaOH, the amount HCI solution (the same solution as before) required to

neutralize it will be

(a) 4mL

(6) 8 mL

(c) 12 mL

(d) 16 mL

Ans. (d)

4. Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic

(b) Analgesic

(c) Antacid

(d) Antiseptic

Ans. (c)

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.

Ans. (a) Zn + H₂SO₄ ⟶       ZnSO₄ + H₂

(b) Mg + 2HCI         ⟶        MgCl₂ + H₂

(c) 2Al + 3H₂SO₄    ⟶        Al₂SO₄ +3H₂

(d) Fe + 2HCI           ⟶      FeCl₂ + H₂

6. Compounds such as alcohol and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.

Ans.  Because alcohol and glucose do not ionise to give hydrogen ions (H⁺). Therefore, these are not regarded as acids. Since these do not ionise, they do not conduct electricity.

Alcohol and Glucose do not produce hydrogen ions like HCl. Thus, they are not acidic.

7. Why does distilled water not conduct electricity, whereas rainwater does?

Ans. Distilled water is not a good conductor of electricity because it is a weak electrolyte and dissociates only slightly to give H⁺ and OH^- ions. However, rainwater contains acidic impurities. Since acids are good conductors of electricity, rainwater conducts electricity.

8. Why do acids not show acidic behaviour in the absence of water?

Ans. The ionisation of acids occurs only in the presence of water. 

HCl + H₂O    ⟶    H₃O⁺ +Cl^-

However, in the absence of water, acids do not ionise to give H30⁺ ions and therefore, do not behave as acids.

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.

Ans. (a) D

(b) C

(c) B

(d) A

(e) E

Increasing order of hydrogen ion concentration: 11<9 <7<4<1

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. In which test tube will the fizzing occur more vigorously and why?

Ans. In test-tube A fizzing will occur more vigorously because HCI is stronger acid than acetic acid and therefore, reacts faster with magnesium ribbon.

11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Ans. Curd is sour in taste and becomes acidic. Therefore, its pH will decrease from pH of 6 to a lower value.

12. A milkman adds a very small amount 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?

Ans. (a) Milkman adds baking soda to milk so that the milk becomes slightly alkaline, Thus, milk will not be converted to acidic curd readily.

(b) This baking soda added milk take a longer time to set to curd because it is alkaline and takes longer time for bacteria to make it acidic.

13. Plaster of Paris should be stored in a moisture-proof container. Explain why?

Ans. When Plaster of Paris comes in contact with moisture (water), it changes to solid hard mass, gypsum. Therefore, its gets wasted. Hence it should be stored in a moisture-proof container.

14. What is a neutralization reaction? Give two examples.

Ans.  Neutralization is the reaction of an acid and base where the effect of base is nullified by an acid or vice- versa. The final product of this reaction is salt and water.

 For example,

(i) HCl + NaOH ⟶     NaCl+ H₂O

(ii) HSO₄ +2KOH ⟶    K₂SO₄ + H₂O

15. Give two important uses of washing soda and baking soda.

Ans. Washing soda is used for

1. Softening of hard water.

2. The manufacture of soap, glass paper, borax, caustic soda, etc.

Baking soda is used for

1. In the preparation of baking powder. Baking powder contain sodium hydrogen carbonate and an acid like tartaric acid or citric acid.

2. Baking soda is used in medicines as antacid. It is used in medicines to remove acidity of the stomach. Therefore, it is important constituent of an antacid.