Acids Bases & Salts CBQs — CBSE Class 10 (2027) with step-wise solutions

Sample Acids Bases & Salts CBQs

1. Question 1MCQ1 mark
   During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to
   • absorb the evolved gas
   • moisten the gas
   • absorb moisture from the gas
   • absorb Cl⁻ ions from the evolved gas
   Show step-wise solution

   1. [0.5 mark] Anhydrous CaCl₂ is a desiccant — strongly absorbs water/moisture.
   2. [0.5 mark] On a humid day, evolved HCl gas would pick up water vapour; we want dry HCl. The CaCl₂ guard tube removes moisture from the gas stream.

   Answer: Absorb moisture from the gas — option (c)

2. Question 2Assertion-Reason1 mark
   Assertion (A): A strip of turmeric paper cannot be used to tell apart dilute hydrochloric acid and common salt solution. Reason (R): Turmeric changes colour only in basic solutions and stays yellow in both acidic and neutral solutions.
   • (A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
   • (B) Both Assertion and Reason are true but Reason is NOT the correct explanation of Assertion.
   • (C) Assertion is true but Reason is false.
   • (D) Assertion is false but Reason is true.
   Show step-wise solution

   1. [1 mark] (A) Both are true and R explains A: since turmeric stays yellow in acidic HCl and in neutral salt solution alike, it gives the same result for both and cannot distinguish them.

   Answer: (A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

3. Question 3Short answer3 marks
   How would you distinguish between baking powder and washing soda by heating?
   Show step-wise solution

   1. [1 mark] Heat each sample separately and pass evolved gas (if any) through lime water.
   2. [0.5 mark] Baking soda/powder: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑. CO₂ turns lime water milky.
   3. [0.5 mark] Washing soda Na₂CO₃·10H₂O → Na₂CO₃ + 10H₂O (water of crystallisation only). No CO₂ released.
   4. [1 mark] Distinction: baking powder gives gas that turns lime water milky; washing soda doesn't.

   Answer: Baking powder releases CO₂ on heating (lime water milky); washing soda just loses water.

4. Question 4Case study4 marks
   Electricity is passed through an aqueous solution of common salt. A substance 'Z' is produced along with the evolution of gases 'X' and 'Y'. Burning matchstick brought near 'Y' burns with a pop sound; 'X' is used for disinfecting drinking water. When 'X' is passed through slaked lime, an insoluble substance 'A' is produced. (a) Name the gases 'X' and 'Y'. (b) Write the balanced chemical equation for the formation of substance 'A'. (c) Observation when (i) blue litmus is added to aqueous Z, (ii) methyl orange is added to Z.
   Show step-wise solution

   1. [1 mark] (a) X = chlorine gas (Cl2) — used to disinfect water; Y = hydrogen gas (H2) — burns with pop sound. Z = sodium hydroxide (NaOH) — the chlor-alkali product.
   2. [1 mark] (b) Cl2 reacting with slaked lime gives bleaching powder: Ca(OH)2 + Cl2 → CaOCl2 + H2O. Substance A = bleaching powder (CaOCl2).
   3. [1 mark] (c)(i) Z (NaOH) is basic, so blue litmus does NOT turn red — colour remains blue.
   4. [1 mark] (c)(ii) Methyl orange in basic medium turns YELLOW.

   Answer: (a) Cl2 and H2; (b) Ca(OH)2 + Cl2 → CaOCl2 + H2O; (c) blue litmus unchanged, methyl orange yellow.

5. Question 5Long answer5 marks
   Mrs Thomas, a laboratory assistant in Kochi, has five unlabelled bottles A to E. They contain, not in this order, dilute hydrochloric acid, dilute acetic acid, dilute sodium hydroxide solution, lime water and distilled water. The two acids were made up to the same concentration. Her students record: Test 1, blue litmus: A and D turn it red; B, C and E do not. Test 2, turmeric paper: B and E turn it red-brown; A, C and D do not. Test 3, carbon dioxide bubbled through B and E: E turns milky; B stays clear. Test 4, zinc granules of the same size added to equal volumes of A and D: A gives brisk effervescence; D gives only a few slow bubbles. (a) Which bottles contain acids and which contain bases? Give the evidence. (1 mark) (b) Identify bottle C, with a reason. (1 mark) (c) Identify bottle E and write the balanced chemical equation for the change seen in Test 3. (1 mark) (d) Identify bottles A and D, giving a reason based on Test 4. (1 mark) (e) Bottle B is dilute sodium hydroxide solution. Write the balanced chemical equation for its reaction with the acid in bottle A, and state the colour of turmeric paper in the mixture if the base is exactly neutralised. (1 mark)
   Show step-wise solution

   1. [1 mark] (a) A and D are acids because they turn blue litmus red; B and E are bases because they turn turmeric paper red-brown.
   2. [1 mark] (b) C is distilled water: it changes neither blue litmus nor turmeric, so it is neutral.
   3. [1 mark] (c) E is lime water, since carbon dioxide turns it milky by forming insoluble calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
   4. [1 mark] (d) A is dilute hydrochloric acid, a strong acid that reacts briskly with zinc; D is dilute acetic acid, a weak acid that gives only a few slow bubbles at the same concentration.
   5. [1 mark] (e) NaOH + HCl → NaCl + H₂O; the exactly neutralised mixture is neutral, so turmeric paper stays yellow.

   Answer: (a) Acids: A and D (turn blue litmus red); bases: B and E (turn turmeric red-brown). (b) C is distilled water (no change with either indicator). (c) E is lime water: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. (d) A is dilute hydrochloric acid (strong acid, brisk reaction); D is dilute acetic acid (weak acid, slow reaction). (e) NaOH + HCl → NaCl + H₂O; turmeric stays yellow.

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