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Class 10 · Science · Chapter 2: Acids, Bases and Salts

Class 10 Science Chapter 2: Important & Previous Year Questions – Bihar Board

Bihar Board · NCERT · CBSE

Only board questions that already carry a verified year are listed — BSEB annual exam questions (previous years) and questions from the board's official model set (marked separately, not an annual exam). No year is guessed. Answers are written by Study With Me, not an official answer key.

← Chapter 2: Acids, Bases and Salts — full notes, video, questions

🏛️ BSEB 2025 · annual secondary exam (previous year questions)

1BSEB 2025 · board paper · Q30 · page 121 mark

Which of the following gases is formed as a result of the reaction of zinc and dilute hydrochloric acid?

  1. (A) N2
  2. (B) H2
  3. (C) SO2
  4. (D) CO2
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Correct answer: (B) H2

Acid + metal → salt + hydrogen. A burning splint makes the gas burn with a pop.

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
2BSEB 2025 · board paper · Q31 · page 121 mark

Which of the following gases turns lime water milky?

  1. (A) CO
  2. (B) CO2
  3. (C) O2
  4. (D) Cl2
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Correct answer: (B) CO2

CO2 reacts with lime water to give a white precipitate of calcium carbonate, so the water looks milky.

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
3BSEB 2025 · board paper · Q32 · page 131 mark

Which among the following is slaked lime?

  1. (A) CaO
  2. (B) CaCO3
  3. (C) Ca(OH)2
  4. (D) Ca
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Correct answer: (C) Ca(OH)2

Slaked lime = calcium hydroxide Ca(OH)2. Quicklime is CaO, and CaCO3 is limestone. Chlorine on dry slaked lime gives bleaching powder.

4BSEB 2025 · board paper · Q33 · page 131 mark

Toothpaste is generally —

  1. (A) Acidic
  2. (B) Salty
  3. (C) Alkaline
  4. (D) None of these
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Correct answer: (C) Alkaline

Tooth decay starts when the mouth pH falls below 5.5. Toothpastes are generally alkaline and neutralise the extra acid in the mouth.

5BSEB 2025 · board paper · Q34 · page 131 mark

Which acid is present in lemon?

  1. (A) Citric acid
  2. (B) Lactic acid
  3. (C) Acetic acid
  4. (D) Formic acid
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Correct answer: (A) Citric acid

Lemons and oranges contain citric acid (NCERT Table 2.3). Lactic acid is in curd, acetic acid in vinegar, formic (methanoic) acid in an ant sting.

6BSEB 2025 · board paper · Q35 · page 131 mark

The pH value of milk of magnesia is —

  1. (A) 4
  2. (B) 5
  3. (C) 8
  4. (D) 10.5
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Correct answer: (D) 10.5

Milk of magnesia (Mg(OH)2) is a mild base, so its pH is well above 7. NCERT Fig. 2.7 shows it at about 10; the closest option is 10.5.

7BSEB 2025 · board paper · Q43 · page 151 mark

The chemical name of lime water is —

  1. (A) Slaked lime
  2. (B) Calcium hydroxide
  3. (C) Quick lime
  4. (D) Calcium oxide
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Correct answer: (B) Calcium hydroxide

Lime water = an aqueous solution of calcium hydroxide Ca(OH)2. «Slaked lime» is its common name, not its chemical name; quicklime/calcium oxide CaO is a different substance.

8BSEB 2025 · board paper · Section B, short answer Q132 marks

Describe how the HCl secreted in the human stomach works. (our translation; the paper prints this question in Hindi)

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  • Glands in the stomach wall secrete dilute hydrochloric acid.
  • It makes the stomach acidic (pH about 1–2), in which the protein-digesting enzyme pepsin works, and it kills bacteria that come in with food.
  • Too much acid causes indigestion/burning; then an antacid such as milk of magnesia (a mild base) neutralises the extra acid.

📄 BSEB model set 2026 (official board model paper, not an annual exam) — important

9BSEB model paper 2026 (not the annual exam) · Q33 · page 121 mark

A solution turns red litmus blue. What would be its pH?

  1. (A) 4
  2. (B) 11
  3. (C) 7
  4. (D) 5
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Correct answer: (B) 11

Red litmus turning blue = a basic solution, so pH above 7. Only 11 fits. (NCERT Exercise Q1 asks the same with options 1, 4, 5, 10 → 10)

10BSEB model paper 2026 (not the annual exam) · Q35 · page 121 mark

Which of the following gases turns lime water milky?

  1. (A) Cl2
  2. (B) O2
  3. (C) NO2
  4. (D) CO2
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Correct answer: (D) CO2

CO2 → a white precipitate of CaCO3: Ca(OH)2 + CO2 → CaCO3 + H2O

11BSEB model paper 2026 (not the annual exam) · Q36 · page 121 mark

Which of the following acids is present in a bee sting?

  1. (A) Methanoic acid
  2. (B) Ethanoic acid
  3. (C) Citric acid
  4. (D) Oxalic acid
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Correct answer: (A) Methanoic acid

The acid in a sting is methanoic (formic) acid. Rubbing a mild base such as baking soda on it gives relief.

12BSEB model paper 2026 (not the annual exam) · Q37 · page 12–131 mark

The chemical formula of 'Plaster of Paris' is —

  1. (A) CaSO4·5H2O
  2. (B) CaSO4·½H2O
  3. (C) CaSO4·2H2O
  4. (D) CaSO4·H2O
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Correct answer: (B) CaSO4·½H2O

Plaster of Paris = calcium sulphate hemihydrate CaSO4·½H2O. CaSO4·2H2O is gypsum.

13BSEB model paper 2026 (not the annual exam) · Q38 · page 131 mark

Acid and base react with each other to form —

  1. (A) Strong base
  2. (B) Strong acid
  3. (C) Salt
  4. (D) None of these
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Correct answer: (C) Salt

Neutralisation: acid + base → salt + water. NaOH + HCl → NaCl + H2O

14BSEB model paper 2026 (not the annual exam) · Section B (Chemistry), short answer Q132 marks

How is sodium carbonate used to soften salty (hard) water?

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Hard (salty) water contains soluble salts of calcium and magnesium. Adding washing soda (Na2CO3) turns them into insoluble carbonates that settle down, so the water becomes soft:

Na2CO3(aq) + CaCl2(aq) → CaCO3(s) + 2NaCl(aq)
Na2CO3(aq) + MgSO4(aq) → MgCO3(s) + Na2SO4(aq)
15BSEB model paper 2026 (not the annual exam) · Section B (Chemistry), short answer Q142 marks

Distilled water is not a conductor of electricity, whereas rain water is. Why?

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Distilled water has no dissolved salts or acids, so it has almost no ions and no current flows. Rain water dissolves gases such as CO2 and SO2 from the air as it falls and forms acids, which give ions (H+ and others) in water. Ions carry the current, so rain water conducts.

16BSEB model paper 2026 (not the annual exam) · Section B (Chemistry), long answer Q195 marks

What are natural indicators? How are acids and bases tested with them?

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Natural indicators come from plants or nature and show an acid or a base by changing their colour (or smell).

IndicatorSourceIn acidIn base
Litmuslichen (Thallophyta)blue → redred → blue
Turmericturmeric rootstays yellowreddish-brown
Red cabbage / beetroot juiceleaves / rootred-pinkgreen-yellow

How to test: take the solution in a test tube and add 2–3 drops of the indicator (or dip indicator paper). Read the colour — if blue litmus turns red the solution is acidic; if red litmus turns blue it is basic. A turmeric stain turns reddish-brown with soap because soap is basic.

Olfactory indicators (onion, vanilla, clove) keep their smell in an acid and lose it in a base.

← Chapter 2: Acids, Bases and Salts — full notes, video, questions