Study Materials

# NCERT Solutions for Class 10th Science

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## Chapter 2. Acids, Bases and Salts

### Text-book Questions

In Text Questions

Page No: 18

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?

Solution:

If the colour of red litmus does not change then it is acid. If the colour of redlitmus changes to blue then it is base. If there is slight change in the colour of red litmus (such as purple) then it is distilled water.

Page No: 22

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

Solution:

Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.

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?

Solution:

Hydrogen gas is usually liberated when an acid reacts with a metal.

Take few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.

H2SO4 + Zn → ZnSO4 + H2 ↑
We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.

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.

Solution:

CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
Calcium Carbonate + Hydrochloric acid → Calcium Chloride + Carbon dioxide + Water.

Page No: 25

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
?

Solution:

When HCl or HNO3 are mixed with water then they dissolve in water to form H+ or H3O+ ions which shows their acidic character. For example just see the following reactions

HCl (aq) → H+ + Cl-

H+ + H2O → H3O+

When alcohols and glucose are mixed with water then they do not dissolve to form ions. Hence they do not show acidic character.

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

Solution:

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

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

Solution:

Dry HCl gas not change the colour of the dry litmus paper because it has no Hydrogen ions (H+) in it.

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

Solution:

Since the process of dissolving an acid in water is exothermic, it is always recommended that acid should be added to water. If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns.

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

Solution:

When an acid is diluted, the concentration of hydronium ions (H3O+) per unit volume decreases. This means that the strength of the acid decreases.

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

Solution:

The concentration of hydroxide ions (OH) would increase when excess base is dissolved in a solution of sodium hydroxide.

Page No: 28

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?

Solution:

A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.

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

Solution:
If the concentration of H+ (aq) ions is increased (>10-7) then the solution become acidic and if the concentration of H+ (aq) ions is decreased (<10-7) then the solution become basic in nature.

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

Solution:

Yes, basic solution also has H+ ions. However, their concentration is less as compared to the concentration of OH- ions that makes the solution basic.

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)?

Solution:

If the soil is acidic and improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with quick lime or slaked lime or chalk.

Page No: 33

1. What is the common name of the compound CaOCl2?
► Bleaching Powder.

2. Name the substance which on treatment with chlorine yields bleaching powder?
► Calcium hydroxide [Ca(OH)2]

3. Name the sodium compound which is used for softening hard water.
► Washing soda (Na2CO3.10H2O)

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

Solution:

When sodium hydrogen carbonate is heated then sodium carbonate and water is formed along with the evolution of carbon dioxide gas.

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