Solution:: Chapter 2: Acids, Bases and Salts

Science Questions and Answers

Page No-18 Questions

Question:

You have three test tubes: one with distilled water, one with an acidic solution, and one with a basic solution. You are given only red litmus paper. How will you identify the contents of each test tube?

Answer: Dip a strip of red litmus paper into each test tube one by one.
Observations and Identification:
If the red litmus paper remains red, the solution is either acidic or neutral (distilled water).
If the red litmus paper turns blue, the solution is basic.
To distinguish between the acidic and neutral (distilled water) solutions, you would need blue litmus paper. However, since you only have red litmus paper, you can compare the two remaining solutions. The one that does not change the red litmus paper and does not react further is distilled water, while the other is acidic.

Page No-22 Questions

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

Curd and sour substances contain acids (such as lactic acid and citric acid). When stored in brass (copper-zinc alloy) or copper vessels, acids react with the metal. This reaction forms toxic salts (like copper acetate or copper chloride), which are harmful to health.

2. Which gas is usually liberated when an acid reacts with a metal?

Hydrogen gas (H₂) is liberated when an acid reacts with a metal.
Example: Zinc (Zn) reacts with Hydrochloric Acid (HCl)
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
How to test for Hydrogen gas?
Bring a burning matchstick or candle near the evolved gas. If hydrogen is present, it burns with a ‘pop’ sound.

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.

Since the gas extinguishes a burning candle, it is carbon dioxide (CO₂). The metal compound A is calcium carbonate (CaCO₃).
Reaction: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Effervescence occurs due to the release of CO₂ gas. To confirm CO₂, pass it through lime water; it turns milky due to the formation of CaCO₃.

Page No-25 Questions

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

HCl and HNO₃ ionize in water to produce H⁺ (or H₃O⁺) ions, which are responsible for their acidic properties. Alcohol and glucose contain hydrogen but do not ionize in water to release H⁺ ions, so they do not exhibit acidic behavior.

Why does an aqueous solution of an acid conduct electricity?

Acids dissociate in water to form free ions (H₃O⁺ and anions like Cl⁻, NO₃⁻, etc.). These ions help in the conduction of electric current through the solution.

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

Dry HCl gas does not produce H⁺ ions in the absence of water. Without ionization, it does not exhibit acidic properties and does not affect dry litmus paper.

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

The process of mixing acid with water is highly exothermic. If water is added to concentrated acid, a large amount of heat is released quickly, causing the mixture to splash out, which may result in burns or glass breakage. Adding acid slowly to water ensures that heat is absorbed safely.

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

Dilution decreases the concentration of H₃O⁺ ions per unit volume. This reduces the acidity of the solution.

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

When excess base dissolves, the concentration of OH⁻ ions increases, making the solution more basic (higher pH).

Page No-28 Questions

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 these is acidic and which one is basic?

Solution A (pH 6) has more hydrogen ion concentration than Solution B (pH 8).
Solution A is acidic because its pH is less than 7.
Solution B is basic because its pH is greater than 7.

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

Higher H⁺(aq) concentration makes the solution more acidic (lower pH). Lower H⁺(aq) concentration makes the solution less acidic or more basic (higher pH).

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

Yes, basic solutions also have H⁺(aq) ions but in very low concentration. A solution is considered basic when the concentration of OH⁻ ions is higher than that of H⁺(aq) ions.

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)?

A farmer would treat the soil with quick lime, slaked lime, or chalk when the soil is too acidic (low pH). These substances are basic and help neutralize excess acidity, improving soil conditions for plant growth.

Page No-28 Questions

What is the common name of the compound CaOCl₂?

Bleaching powder

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

Slaked lime (Ca(OH)₂)

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

Washing soda (Na₂CO₃·10H₂O)

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

Sodium hydrogenc arbonate (NaHCO₃) decomposes to form sodium carbonate, carbon dioxide, and water.
Equation: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

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

CaSO₄·½H₂O + 3/2 H₂O → CaSO₄·2H₂O

Exercise

Here are all the answers along with the questions:

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

(a) 1
(b) 4
(c) 5
(d) 10 ✅
Answer: A solution that turns red litmus blue is basic. Since pH 10 is basic, the correct answer is (d) 10.

2. A solution reacts with crushed eggshells to give a gas that turns lime-water milky. The solution contains:

(a) NaCl
(b) HCl ✅
(c) LiCl
(d) KCl
Answer: Eggshells contain calcium carbonate (CaCO₃), which reacts with hydrochloric acid (HCl) to produce carbon dioxide (CO₂). CO₂ turns lime water milky.
Equation: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

3. 10 mL of a solution of NaOH is found to be completely neutralized by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralize it will be:

(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL ✅
Answer: Since the volume of NaOH is doubled, the required volume of HCl is also doubled. Correct answer: (d) 16 mL.

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

(a) Antibiotic
(b) Analgesic
(c) Antacid ✅
(d) Antiseptic
Answer: Antacids neutralize excess stomach acid, relieving indigestion. Correct answer: (c) Antacid.

5. Write word equations and balanced chemical equations for the following reactions:

(a) Dilute sulphuric acid reacts with zinc granules.
Word equation: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Balanced equation: Zn + H₂SO₄ → ZnSO₄ + H₂
(b) Dilute hydrochloric acid reacts with magnesium ribbon.
Word equation: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
Balanced equation: Mg + 2HCl → MgCl₂ + H₂
(c) Dilute sulphuric acid reacts with aluminium powder.
Word equation: Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen
Balanced equation: 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
(d) Dilute hydrochloric acid reacts with iron filings.
Word equation: Iron + Hydrochloric acid → Iron chloride + Hydrogen
Balanced equation: Fe + 2HCl → FeCl₂ + H₂

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

Activity:
Take two beakers with distilled water. Dissolve glucose in one and alcohol in the other. Dip two electrodes and connect them to a bulb and a battery. The bulb does not glow, showing no conduction of electricity. This proves that glucose and alcohol do not produce hydrogen ions in water, so they are not acids.

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

Distilled water is pure and does not contain any ions, so it does not conduct electricity. Rainwater contains dissolved acids (like carbonic acid from CO₂), which provide ions, making it conductive.

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

Acids release hydrogen ions (H⁺) only in the presence of water. Without water, they remain in molecular form and do not exhibit acidic properties.

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? D (pH = 7)
(b) Strongly alkaline? C (pH = 11)
(c) Strongly acidic? B (pH = 1)
(d) Weakly acidic? A (pH = 4)
(e) Weakly alkaline? E (pH = 9)
Increasing order of hydrogen-ion concentration: C < E < D < A < B

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

Fizzing will occur more vigorously in test tube A because hydrochloric acid is a strong acid and ionizes completely, releasing more hydrogen ions (H⁺), leading to a faster reaction with magnesium. Acetic acid is weak and ionizes partially, so the reaction is slower.

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

The pH will decrease (become more acidic) because lactic acid is produced during the fermentation of milk.

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? To prevent milk from becoming too acidic and spoiling quickly.
(b) Why does this milk take a long time to set as curd? Because the slightly alkaline nature slows down the action of lactic acid bacteria.

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

Plaster of Paris (CaSO₄·½H₂O) absorbs moisture and turns into gypsum (CaSO₄·2H₂O), making it unusable.

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

A reaction in which an acid and a base react to form salt and water.
Examples:
HCl + NaOH → NaCl + H₂O
H₂SO₄ + Ca(OH)₂ → CaSO₄ + 2H₂O

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

Washing soda (Na₂CO₃):
Used in laundry for cleaning clothes.
Softens hard water.
Baking soda (NaHCO₃):
Used in baking as a leavening agent.
Used as an antacid.