ScienceCBSE Class 10 · Board exam 2025, set 31/3/1 · 3 marks · Short answer

CBSE Class 10 Science 2025: question 28(a)

Set in the CBSE Class 10 board exam of 2025, paper 31/3/1, section C, question 28(a), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q28(a) · 3 marks · Short answerAcids, Bases and Salts

Common salt is an important raw material for various chemicals of daily use. State in brief the method of preparation of (i) Sodium hydroxide, and (ii) Sodium hydrogen carbonate from common salt. Write balanced chemical equations of the reactions that occur.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/3/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

The name 'chlor-alkali' tells you the products of passing electricity through brine.

Hint 2

For baking soda, brine reacts with two gases – one of them is ammonia.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(i) Sodium hydroxide: electricity is passed through brine (aqueous NaCl) – the chlor-alkali process. Chlorine forms at the anode, hydrogen at the cathode and NaOH solution near the cathode.
½ mark
$2 N a C l (a q) + 2 H_{2} O (l) \to 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)$
1 mark
(ii) Sodium hydrogen carbonate: a cold solution of brine is treated with ammonia and carbon dioxide; sodium hydrogen carbonate separates out and ammonium chloride forms.
½ mark
$N a C l + H_{2} O + C O_{2} + N H_{3} \to N H_{4} C l + N a H C O_{3}$
1 mark

Answer: (i) Electrolysis of brine: $2 N a C l + 2 H_{2} O \to 2 N a O H + C l_{2} + H_{2}$ ; (ii) $N a C l + H_{2} O + C O_{2} + N H_{3} \to N H_{4} C l + N a H C O_{3}$

Where the marks are

Method for NaOH – electrolysis of brine (½ mark)
Balanced chlor-alkali equation (1 mark)
Method for NaHCO₃ – brine with NH₃ and CO₂ (½ mark)
Balanced equation (1 mark)

Revise the chapter

Chapter: Acids, Bases and Salts (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Acids, Bases and Salts, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/3/1: every question, by section.

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CBSE Class 10 Science 2025: question 28(a)

Hints

Step-by-step solution

Revise the chapter

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