NCERT Solutions of class 10 Science ch-2 Acids,Bases and Salts

Here I have provided you NCERT Solutions for Class 10 Science Chapter 2 Acids, Bases and Salts. By going through Acids, Bases and Salts Question Answers you will acquire a better command on this chapter. I hope that this will certainly help you in your studies and examinations!

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?

Sol.

First of all, we will put one strip of red litmus paper in each test tube. In one of the test tubes, the litmus paper will turn blue. It is sure that this test tube contains basic solution. The remaining two test tubes contain distilled water and acidic solution in both of which red litmus remains red.

Now, we will pour out some basic solution from the test tube so that only a little amount remains in it. Then, we will pour the contents of one of the remaining test tubes into the test tube containing basic solution. Two things may happen :

(a) The litmus may turn red again : This indicates that the second test tube contains acidic substance. And the other one contains distilled water.

(b) The litmus may remain blue : This indicates that the poured liquid is distilled water. And the other one is the acidic solution.

Page Number – 22

Q.1 Why should curd and sour substances not be kept in brass and copper vessels?

Sol.

Curd and sour substances contain acids. Acids react with metals to give salt and hydrogen gas. So, if such substances are kept in copper container, the acid will react and the container will corroded.

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?

Sol.

When an acid reacts with any metal, salt and hydrogen gas are formed.

Metal + Acid → Salt + Hydrogen gas

Q.3 A Metal compound 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

Sol. CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + CO₂ (g) + H₂O (l)

Page Number – 25

Q.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?

Sol.

HCl, HNO3, etc. have releasable H⁺ ion. When dissolved in water, the H⁺ ion gets separated and shows acidic character. In alcohol or glucose, there is no releasable H⁺ ion.

Q.2 Why does an aqueous solution of an acid conduct electricity?

Sol.

The presence of hydrogen (H⁺) or hydronium (H₃O⁺) ions in the aqueous solution of an acid are responsible for conducting electricity.

Q.3 Why does dry HCl gas not change the colour of the dry litmus paper?

Sol.

Dry HCl does not dissociate to give H+ ions. So, it does not show acidic character.

Q.4 While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Sol.

The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.

Q.5 How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Sol. Concentration of H3O+ ion decreases.

Q.6 How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide ?

Sol. Concentration of OH– group increases.

Page Number – 28

Q.1 You have two solutions, A and B. The pH solution A is 6 and pH of solution B is 8. While solution has more hydrogen ion concentration. Which of this is acidic and which one is basic ?

Sol.

The solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.

Q.2 What effect does the concentration of H+(aq) ions have on the nature of the solution?

Sol.

As the concentration of H+ ions increases the solution becomes more acidic.

Q.3 Do basic solutions also have H+(aq) ions? If yes, then why are these basic?

Sol. This H+ ion comes from water.

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)?

Sol.

If the farmer finds his soil turned acidic, he should use the bases to neutralise it.

Page Number – 33

Q.1 What is the common name of the compound CaOCl2?

Sol. Bleaching powder.

Q.2 Name the substance which on treatment with chlorine yields bleaching powder.

Sol. Dry slaked lime [Ca(OH)2]

Q.3 Name the sodium compound which is used for softening hard water.

Sol. Washing soda i.e., sodium carbonate (Na₂CO₃.10H₂O).

Q.4 What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Sol.

Heating sodium hydrocarbonate yields sodium carbonate and carbon dioxide gas is liberated in the process.

Q.5 Write an equation to show the reaction between Plaster of Paris and water.

Sol.

Back Exercise Questions

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

(a) 1 (b) 4 (c) 5 (d) 10

Sol. (d) 10

Q.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

Sol. (b) HCl

Q.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

Sol. (d) 16 mL

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

(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Sol. (c) Antacid

Q.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.

Sol.

(a) Dil. Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)

(b) Dil. Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)

2Al (s) + 3H2SO4 (aq) → Al2 (SO4)3 (aq) + 3H2 (g)

(d) Dil. Hydrochloric acid + Iron → Ferric chloride + Hydrogen gas

2Fe (s) + 6HCl (aq) → 2FeCl3 (aq) + 3H2 (g)

Q.6 Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.

Sol.

The apparatus is set as shown in figure. Solutions of glucose is added and the switch is turned on. The bulb does not glow. This means glucose does not dissociate into ions on dissociation. So, glucose is not an acid. Similarly, alcohol is also not an acid.

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

Sol.

Distilled water does not dissociate into ions. So, does not conduct electricity. Rain water has dissolved in it which becomes carbonic acid.

CO2+H2O→H2CO3

This carbonic acid dissociate into ions.

H2CO3+2H2O→(H3O)²+(CO3)2+

These ions are responsible for electrical conductivity rain water.

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

Sol.

In absence of water, acids do not dissociated they do not show acidic behaviour.

Q.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.

Sol.

(a) (D), (b) C, (c) B, (d) A, (e) E.

Increasing order of H+ concentration :

C(11) < E (9) < D (7) < A (4) < B (1).

Q.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 ?

Sol.

In test tube A, fizzing occur more vigorously because HCl is a strong acid and dissociate more.

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

Sol. Curd is sour which means its pH will decrease from 6.

Q.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 to slightly alkaline?

(b) Why does this milk take a long time to set as curd?

Sol.

(a) Baking soda is alkaline which causes the alkalinity of milk.

(b) Curd needs acidic condition to set.

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

Sol.

Plaster of Paris easily absorbs water and forms hard gypsum. Thus, if Plaster of Paris is not kept in a moisture-proof container, then all Plaster of Paris will get converted into gypsum.

Q.14 What is a neutralisation reaction? Give two examples.

Sol.

The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.
NaOH + HCl → NaCl + H₂O
HNO₃ + KOH → KNO₃ + H₂O
Q.15 Give two important uses of washing soda and baking soda.

Sol.

Uses of Washing Soda –

(i) Sodium carbonate (washing soda) is used in glass soap and paper industries.

(ii) It is used in the manufacture of sodium compound such as borax.

Uses of Baking Soda –

(i) Sodium hydrogencarbonate is also an mgredient antacids. Being alkaline, it neutralises excess acid in stomach and provides relief.

(ii) It is also used in soda–acid fire extinguishers.