Acids, Bases and Salts: Class 10 Important Questions with Answers

Case-based questions (4 marks each)

(i) Arrange P, Q, R, S in increasing order of pH. (1) (ii) Which one could be vinegar and which could be HCl? Give a reason. (1) (iii) What happens to the colour of phenolphthalein in R and in P? What will be seen if equal amounts of P and R that exactly neutralise are mixed and tested with universal indicator? (2)

(i) Name X, Y and Z. (1) (ii) Write the equation for the reaction that makes the cake soft and spongy. (1) (iii) Why does the cake taste bitter with Y alone? Give two other uses of Y. (2)

(i) Name W and V with formulae. (1) (ii) Write the equation for the setting of the cast. (1) (iii) Why is W called a hemihydrate, and why must W be stored in a moisture-proof bag? (2)

(i) Is this rain normal? Give the pH limit. (1) (ii) Why are the fish dying? (1) (iii) Explain with an equation why marble (CaCO₃) is damaged, and suggest how the farmers can correct their acidic soil. (2)

20 MCQs (1 mark each)

1. Phenolphthalein in a basic solution is
   (a) colourless (b) pink (c) red (d) yellow
2. Methyl orange in an acid turns
   (a) yellow (b) orange (c) red (d) blue
3. Which is an olfactory indicator?
   (a) litmus (b) turmeric (c) vanilla essence (d) methyl orange
4. Gas evolved when zinc reacts with dilute HCl:
   (a) Cl₂ (b) H₂ (c) O₂ (d) CO₂
5. Lime water turns milky due to the formation of
   (a) Ca(OH)₂ (b) CaCO₃ (c) Ca(HCO₃)₂ (d) CaO
6. A solution with pH 2 compared to pH 5 has
   (a) less H⁺ (b) more H⁺ (c) same H⁺ (d) more OH⁻
7. Which salt gives a basic solution in water?
   (a) NaCl (b) NH₄Cl (c) Na₂CO₃ (d) CuSO₄
8. Which salt gives an acidic solution?
   (a) KNO₃ (b) NH₄Cl (c) NaHCO₃ (d) CH₃COONa
9. Gas set free at the anode in the chlor-alkali process:
   (a) H₂ (b) O₂ (c) Cl₂ (d) HCl
10. Bleaching powder is made by passing Cl₂ over
    (a) quick lime (b) dry slaked lime (c) limestone (d) gypsum
11. Number of water molecules in washing soda:
    (a) 2 (b) 5 (c) 7 (d) 10
12. Gypsum is converted to PoP at
    (a) 273 K (b) 373 K (c) 473 K (d) 573 K
13. Acid in curd:
    (a) citric (b) lactic (c) oxalic (d) tartaric
14. Bee sting is treated with
    (a) vinegar (b) lemon juice (c) baking soda (d) HCl
15. Tooth decay starts when mouth pH falls below
    (a) 7.4 (b) 6.5 (c) 5.5 (d) 3.5
16. Dry HCl gas does not turn dry blue litmus red because
    (a) it is a base (b) it has no H atoms (c) no H⁺ ions form without water (d) litmus is dry only
17. Which is an antacid?
    (a) NaOH (b) Mg(OH)₂ (c) Ca(OH)₂ (d) KOH
18. The colour of anhydrous copper sulphate is
    (a) blue (b) green (c) white (d) black
19. 2NaHCO₃ on heating gives
    (a) Na₂CO₃ + H₂O + CO₂ (b) NaOH + CO₂ (c) Na₂O + CO₂ (d) NaCl + CO₂
20. Which oxide reacts with NaOH to give salt and water?
    (a) CaO (b) MgO (c) CO₂ (d) Na₂O

Answer key

Why: 6 → lower pH = more H⁺ · 7 → weak acid + strong base · 8 → strong acid + weak base · 17 → mild base, safe to swallow (NaOH, KOH are corrosive) · 20 → CO₂ is an acidic oxide.

Assertion–Reason (10)

(a) Both A and R true, R explains A · (b) Both true, R does not explain A · (c) A true, R false · (d) A false, R true

1. A: Aqueous solution of HCl conducts electricity. R: HCl gives H⁺ and Cl⁻ ions in water. → (a)
2. A: Glucose solution is acidic. R: Glucose contains hydrogen. → (d) it gives no H⁺ ions
3. A: Acid should be added to water, not water to acid. R: Dilution of acids is highly exothermic. → (a)
4. A: pH of NaCl solution is 7. R: NaCl is formed from a strong acid and a strong base. → (a)
5. A: Na₂CO₃ solution turns red litmus blue. R: Washing soda is used to remove permanent hardness of water. → (b)
6. A: Plaster of Paris is stored in moisture-proof containers. R: PoP sets into a hard mass of gypsum on absorbing water. → (a)
7. A: All bases are alkalis. R: An alkali is a base that dissolves in water. → (d) Cu(OH)₂ is not an alkali
8. A: Copper sulphate crystals turn white on heating. R: CuSO₄ decomposes into CuO on mild heating. → (c) water of crystallisation is lost
9. A: Excess CO₂ makes milky lime water clear again. R: Soluble calcium hydrogencarbonate is formed. → (a)
10. A: Acetic acid is a weak acid. R: Vinegar contains acetic acid. → (b)

Give reasons (2 marks each)

1. Antacids are taken during indigestion.

Indigestion means too much HCl in the stomach. Antacids such as Mg(OH)₂ are mild bases that neutralise the excess acid and give relief. 2

2. Toothpastes are basic.

Bacteria make acids in the mouth; when pH falls below 5.5 enamel corrodes. Basic toothpaste neutralises the excess acid and prevents tooth decay. 2

3. A burning candle goes out in the gas from NaHCO₃ + acid.

The gas is CO₂, which does not support burning and is heavier than air; it cuts off oxygen. Used in soda-acid fire extinguishers. 2

4. HCl is a strong acid but acetic acid is weak, though both are of the same concentration.

HCl ionises completely in water and gives many H⁺ ions; acetic acid ionises only partly and gives few H⁺ ions. 2

5. Rain water conducts electricity, but distilled water does not.

Rain water dissolves gases like CO₂ from air, forming an acid that gives ions; distilled water has almost no ions. 2