E X E R C I S E S CHAPTER 2 Acids, Bases and Salts SCIENCE NCERT TEXTBOOK (CBSE) CLASS 10:

 

 E X E R C I S E S

CHAPTER 2 Acids, Bases and Salts


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

  • The solution is alkaline, as it turns red litmus paper blue. Alkalis generally have a pH above 7.

Answer: (d) 10

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

  • The gas that turns lime-water milky is carbon dioxide (CO₂), which indicates the solution is acidic, and the reaction likely involves an acid such as hydrochloric acid (HCl) reacting with calcium carbonate (in egg-shells).

Answer: (b) HCl

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:

  • The amount of HCl required is directly proportional to the volume of NaOH. Since 10 mL NaOH requires 8 mL HCl, 20 mL NaOH will require double the amount of HCl.

Answer: (d) 16 mL

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

  • Antacids are used to neutralize excess stomach acid, thus treating indigestion.

Answer: (c) Antacid

5. Write word equations and then balanced equations for the reaction taking place when:

(a) Dilute sulfuric acid reacts with zinc granules:

  • Word equation: Zinc + Sulfuric acid → Zinc sulfate + Hydrogen gas
  • Balanced equation:
    Zn(s)+H2SO4(aq)→ZnSO4(aq)+H2(g)\text{Zn} (s) + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{ZnSO}_4 (aq) + \text{H}_2 (g)

(b) Dilute hydrochloric acid reacts with magnesium ribbon:

  • Word equation: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen gas
  • Balanced equation:
    Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\text{Mg} (s) + 2\text{HCl} (aq) \rightarrow \text{MgCl}_2 (aq) + \text{H}_2 (g)

(c) Dilute sulfuric acid reacts with aluminium powder:

  • Word equation: Aluminium + Sulfuric acid → Aluminium sulfate + Hydrogen gas
  • Balanced equation:
    2Al(s)+6H2SO4(aq)→2Al2SO4(aq)+3H2(g)

(d) Dilute hydrochloric acid reacts with iron filings:

  • Word equation: Iron + Hydrochloric acid → Iron chloride + Hydrogen gas
  • Balanced equation:
    Fe(s)+2HCl(aq)→FeCl2(aq)+H2(g)\text{Fe} (s) + 2\text{HCl} (aq) \rightarrow \text{FeCl}_2 (aq) + \text{H}_2 (g)

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

  • Activity:
    Take equal amounts of glucose and ethanol (alcohol) and test them with litmus paper. They will not change the color of the litmus paper, indicating that they do not have acidic properties like acids such as hydrochloric acid (HCl).
    This shows that although alcohols and glucose contain hydrogen, they do not release H⁺ ions in water like acids do.

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

  • Distilled water is pure water, and it does not contain any dissolved ions (like salts) to carry electric current, so it does not conduct electricity.
  • Rainwater, on the other hand, contains dissolved impurities (like CO₂ forming carbonic acid, or dissolved salts), which makes it capable of conducting electricity.

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

  • Acidic behavior (such as turning litmus paper red) is due to the presence of hydrogen ions (H⁺) in the solution. In the absence of water, acids do not ionize to release H⁺ ions, and thus they do not show acidic behavior.

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?
    A solution with a pH of 7 is neutral.
    Answer: D (pH 7)

  • (b) Strongly alkaline?
    A solution with a pH above 10 is strongly alkaline, but the highest pH provided here is 11, so that is strongly alkaline.
    Answer: C (pH 11)

  • (c) Strongly acidic?
    A solution with a pH below 3 is strongly acidic, but the lowest pH here is 1, which is strongly acidic.
    Answer: B (pH 1)

  • (d) Weakly acidic?
    A solution with a pH between 4 and 6 is weakly acidic.
    Answer: A (pH 4)

  • (e) Weakly alkaline?
    A solution with a pH between 8 and 10 is weakly alkaline.
    Answer: E (pH 9)

Order of pH in increasing hydrogen-ion concentration:

  • B (pH 1) → A (pH 4) → D (pH 7) → E (pH 9) → C (pH 11)

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. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

  • Fizzing occurs due to the release of hydrogen gas. Hydrochloric acid (HCl) is a stronger acid than acetic acid (CH₃COOH), so it will react more vigorously with magnesium, producing more hydrogen gas.
    Answer: Test tube A (HCl)

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

  • As milk turns into curd, lactic acid is produced by the fermentation of lactose, which decreases the pH of the milk, making it more acidic.
    Answer: The pH will decrease (become more acidic) as milk turns into curd.

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?

  • Baking soda (NaHCO₃) is a weak base, so it will neutralize some of the natural acidity of the milk, making the pH slightly alkaline.
    Answer: Baking soda acts as a base and raises the pH of the milk.

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

  • Curdling of milk requires an acidic environment. The alkaline pH created by baking soda slows down the fermentation process, thus delaying curdling.
    Answer: The milk becomes slightly alkaline, delaying the acidification needed for curdling.

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

  • Plaster of Paris (CaSO₄·½H₂O) reacts with moisture in the air to convert back into gypsum (CaSO₄·2H₂O). This reaction will ruin the plaster of Paris.
    Answer: It absorbs moisture and hardens into gypsum, making it unusable.

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

  • A neutralization reaction is the reaction between an acid and a base to form water and a salt.
    • Example 1:
      HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)
    • Example 2:
      H2SO4(aq)+NaOH(aq)→Na2SO4(aq)+H2O(l)\text{H}_2\text{SO}_4 (aq) + \text{NaOH} (aq) \rightarrow \text{Na}_2\text{SO}_4 (aq) + \text{H}_2\text{O} (l)

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

  • Washing soda (Na₂CO₃) is used for:

    1. Softening water in washing processes.
    2. Cleaning agents for removing grease and stains.

  • Baking soda (NaHCO₃) is used for:

    1. Leavening agent in baking to make dough rise.
    2. Antacid for treating heartburn.

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