NCERT Solutions Science Chapter 2 Acids and Bases for Class 10th

Page Number: 18

Question 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 ?
Answer:
(i) Put the red litmus paper in all the test tubes, turn by turn. The solution which turns red litmus to blue will be a basic solution. The blue litmus paper formed here can now be used to test the acidic solution.
(ii) Put the blue litmus paper obtained above in the remaining two test-tubes, turn-by-turn. The solution which turns the blue litmus paper to red will be the acidic solution.
(iii) The solution which has no effect on any litmus paper will be neutral and hence it will be distilled water.

Page Number: 22

Question 1
Why should curd and sour substances not be kept in brass and copper vessels ?
Answer:
Curd and sour substances should not be kept in brass and copper vessels because these and other sour food-stuffs contain acids which can react with the metal of the vessel to form poisonous metal compounds which can cause food poisoning and affect our health adversely.

Question 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 ?
Answer:
(i) Hydrogen (H₂) gas is liberated when an acid reacts with a metal.
(ii) Illustration : Set up the apparatus as shown in the given figure. Take some zinc granules in the test tube. Add about 5 mL dilute hydrochloric acid slowly. Soon the reaction between zinc and hydrochloric acid starts and hydrogen gas is evolved.

Metal + Acid → Salt + Hydrogen gas

(iii) Test for H₂ gas :
H₂ gas is not soluble in water. When passed through soap solution, it gets trapped into bubbles.
Bring a burning candle near the soap bubble filled with gas. The soap bubble bursts and hydrogen gas burns with a pop sound.

CaCO₃ (s) + 2HCl (Aq) → CaCl₂( Aq) + CO₂ (g) + H₂O (l)

Page number – 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: Release of H⁺ ion in water will make a compound acidic or non-acidic. Acids are the substance which upon dissociating with water results in production of Hydrogen ions. Some compounds show acidic character as they dissociate in the aqueous solution which results in the production of hydrogen ions (acids like HCl, HNO₃).

Compounds similar to glucose or alcohol do contain hydrogen element but they do not show signs of acidic nature. The fact that the hydrogen in them will not separate as like the hydrogen in the acids. They will not separate to become hydrogen ions, on dissolving in the water.

Hence dissociation of hydrogen gas will decide the acidic or non-acidic nature of a compound.

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

Solution: Charged particles are responsible for the conductance of electricity in an acid. These charged particles called as ions are the reason behind conductance of electricity in acid.

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

Solution: HCL does not give out Hydrogen ions, therefore HCL does not show any acidic behaviour and colour of the litmus paper remain the same on reacting with HCl gas.

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

Solution: While diluting an acid, it is recommended that the acid should be added to water and not water to the acid because if water is added to concentrated acid, it release huge amount of heat which may result in explosion and can cause acid burns o face, clothes and body parts. Hence it is safe to add acid to water but not water to acid.

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

Solution: When acid is added to water there will be a fixed amount of hydronium present in the fixed volume of solution. If we dilute the solution hydronium ion per volume of solution decrease, this in-turn decreases Hydronium concentration in the solution.

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

Solution: When base is dissolved in sodium hydroxide solution its hydroxide ions increase but it will reach saturation at some point. After saturation point hydroxide ion concentration is not affected even after adding base further.

 Page number – 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: In order to find the hydrogen ion concentration, we can use the rule that states, “The pH of any solution is inversely proportional to the hydrogen ion concentration”. Therefore, it means that the solution that has a lower pH number will have a higher hydrogen ion concentration. Hence, solution A will have a higher hydrogen ion concentration. In addition, solution B will be basic and A will be acidic.

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

Solution: Hydrogen ion concentration decides the nature of the solution. If Hydrogen ion concentration increase then solution turn acidic and similarly if Hydrogen ion concentration decreases then solution turn basic.

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

Solution: Basic solutions has H⁺ ions, but hydroxide ions present in basic solution are more in basic solution. Hence Hydroxide ions turn solution to 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 in nature (PH below 7) then such field should be treated with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).

Page number – 33

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

Solution: Common name of CaOCl₂ is bleaching powder.

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

Solution: The substance which on treatment with chlorine yields bleaching powder is Calcium hydroxide.

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

Solution: Sodium carbonate is the compound which is used for softening hard water.

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

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

EX- textbook

Question 1
A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Answer:
(d) 10

Question 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
Answer:
(b) HCl

Question 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 of HC1 solution (the same solution as before) required to neutralise it will be
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL
Answer:
(d) 16 mL

Question 4
Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Answer:
(c) Antacid

Question 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 filing
Answer:
(a) Zinc + dilute sulphuric acid → Zinc sulphate + Hydrogen
Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)

(b) Magnesium ribbon + dil. Hydrochloric acid → Magnesium chloride + Hydrogen
Mg (s) + 2 HCl (aq) → MgCl₂ (aq) + H₂ (g)

(c) Aluminium powder + dil. Sulphuric acid > Aluminium sulphate + Hydrogen
2Al (s) + 3H₂SO₄ (aq) → Al₂ (SO₄)₃ (aq) + 3H₂ (g)

(d) Iron filings + Dilute hydrochloric acid > Ferric chloride + Hydrogen
2Fe (s) + 6HCl (aq) → 2FeCl₃ (aq) + 3H₂ (g)

Question 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 filing
Answer:
(a) Zinc + dilute sulphuric acid → Zinc sulphate + Hydrogen
Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)

(b) Magnesium ribbon + dil. Hydrochloric acid → Magnesium chloride + Hydrogen
Mg (s) + 2 HCl (aq) → MgCl₂ (aq) + H₂ (g)

(c) Aluminium powder + dil. Sulphuric acid > Aluminium sulphate + Hydrogen
2Al (s) + 3H₂SO₄ (aq) → Al₂ (SO₄)₃ (aq) + 3H₂ (g)

(d) Iron filings + Dilute hydrochloric acid > Ferric chloride + Hydrogen
2Fe (s) + 6HCl (aq) → 2FeCl₃ (aq) + 3H₂ (g)

(iv) Now pour alcohol in the beaker and switch on the current.
(v) The bulb does not glow.
(vi) Repeat the experiment with glucose. The bulb does not glow in this case also.
(vii) This means no ions or H+ ions are present in the solution.
This shows that alcohols and glucose are not acids.

Question 7
Why does distilled water not conduct electricity, whereas rainwater does ?
Answer:
Distilled water does not conduct electricity because it does not contain any ionic compound (like acids, bases or salts) dissolved in it.
Rainwater, while falling to the earth through the atmosphere, dissolves an acidic gas carbon dioxide from the air and forms carbonic acid (H₂CO₃). Carbonic acid provides hydrogen ions, H⁺ (aq) and carbonate ions, CO(aq)₃²to rainwater. Hence, due to the presence of carbonic acid which provides ions to rainwater, the rainwater conducts electricity.

Question 8
Why do acids not show acidic behaviour in the absence of water ?
Answer:
The acidic behaviour of acids is due to the presence of hydrogen ions, [H⁺ (aq) ions], in them. The acid produces hydrogen ions only in the presence of water. So in the absence of water, an acid will not form hydrogen ions and hence will not show its acidic behaviour.

Question 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.
Answer:

(a) D
(b) C
(c) B
(d) A
(e) E
Increasing order of hydrogen ion concentration
11 < 9 < 7 < 4 < 1
i. e., C < E < D < A < B

Question 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 ?
Answer:
Fizzing will occur more vigorously in test tube A. Hydrochloric acid (HCl) is a strong acid whereas acetic acid (CH₃COOH) is a weak acid. Being strong acid, the hydrochloric acid solution contains a much greater amount of hydrogen ions in it due to which the fizzing will occur more vigorously in test tube A (containing hydrochloric acid). The fizzing is due to the evolution of hydrogen gas which is formed by the action of acid on the magnesium metal of magnesium ribbon.

Question 11
fresh milk has a pH of 6. How do you think the pH will change as it turns into curd ? Explain your answer.
Answer:
pH of milk falls below 6 as it turns into curd due to the formation of lactic acid during this process. Lactic acid present in it reduces its pH value.

Question 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 6 to slightly alkaline ?
(b) Why does this milk take a long time to set as curd ?
Answer:
(a) Milk is made slightly alkaline so that it may not get sour easily due to the formation of lactic acid in it.
(b) The alkaline milk takes a longer time to set into curd because the lactic acid being formed has to first neutralise the alkali present in it.

Question 13
Plaster of Paris should be stored in a moisture proof container. Explain why?
Answer:
Plaster of Paris should be stored in a moisture proof container because the presence of moisture can cause slow setting of plaster of Paris by bringing about its hydration. This will make the plaster of Paris useless after sometime.

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

Answer:
The reaction between an acid and a base to form salt and water is called a neutralisation reaction.
Examples:

Question 15
Give two important uses of washing soda and baking soda.
Answer:
Uses of washing soda :
(i) Washing soda is used in glass, soap and paper industries.
(ii) It is used for removing permanent hardness of water.

Uses of baking soda :
(i) Baking soda is used as an antacid in medicines to remove acidity of the stomach.
(ii) Baking soda is used for making baking powder (used in making cakes, bread, etc.).