Read the following passage and answer the questions that follow : In the given diagram, when electricity is passed through an aqueous solution of a common salt, a substance ‘Z’ is produced, along with the evolution of gases ‘X’ and ‘Y’. When a burning matchstick is brought near the gas ‘Y’, it burns with a pop sound, whereas ‘X’ is used for disinfecting drinking water. When ‘X’ is passed through slaked lime, an insoluble substance ‘A’ is produced. (a) Write the names of the gases ‘X’ and ‘Y’. (b) Write the balanced chemical equation for the formation of substance ‘Z’. (c)(i) What happens : (I) If a drop of red litmus solution is added to the aqueous solution of substance ‘Z’ ? (II) If CO_2 gas is passed through ammoniacal solution of aqueous NaCl ? (c)(ii) Identify the substance ‘A’ and write the balanced chemical equation of the reaction which takes place in its preparation.

ScienceCBSE Class 10 · Board exam 2026, set 31/2/1 · 4 marks · Case-based

CBSE Class 10 Science 2026: question 28

Set in the CBSE Class 10 board exam of 2026, paper 31/2/1, section B, question 28, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q28 · 4 marks · Case-basedCompetency-basedAcids, Bases and Salts

Read the following passage and answer the questions that follow :

In the given diagram, when electricity is passed through an aqueous solution of a common salt, a substance ‘Z’ is produced, along with the evolution of gases ‘X’ and ‘Y’. When a burning matchstick is brought near the gas ‘Y’, it burns with a pop sound, whereas ‘X’ is used for disinfecting drinking water. When ‘X’ is passed through slaked lime, an insoluble substance ‘A’ is produced.

(a)
Write the names of the gases ‘X’ and ‘Y’.
1 mark
(b)
Write the balanced chemical equation for the formation of substance ‘Z’.
1 mark
(c)(i)
What happens :
(I) If a drop of red litmus solution is added to the aqueous solution of substance ‘Z’ ?
(II) If $C O_{2}$ gas is passed through ammoniacal solution of aqueous NaCl ?
2 marks
OR
(c)(ii)
Identify the substance ‘A’ and write the balanced chemical equation of the reaction which takes place in its preparation.
2 marks

Figure in the paper: Electrolytic cell for brine with a membrane (dotted vertical line) in the middle. Anode (+) electrode on the left with gas X leaving at top-left; Cathode (−) electrode on the right with gas Y leaving at top-right. Brine enters at the left; Z leaves at the right. Near the membrane arrows show Cl⁻ and OH⁻ moving left, and Na⁺ and H⁺ moving right. Membrane labelled at bottom. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 31/2/1.

Hints

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

Hint 1

Electrolysis of brine is called the chlor-alkali process; name its three products.

Hint 2

Which gas gives a pop sound and which is used to disinfect water?

Hint 3

Chlorine with slaked lime gives a common bleaching substance.

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.

(a) X: chlorine gas ( $C l_{2}$ ), released at the anode and used to disinfect water.
½ mark
(a) Y: hydrogen gas ( $H_{2}$ ), released at the cathode; it burns with a pop sound.
½ mark
(b) Z is sodium hydroxide, formed in the chlor-alkali process:
$2 N a C l (a q) + 2 H_{2} O (l) \to 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)$
1 mark
(c)(i)(I) Z (NaOH) is a base, so the red litmus solution turns blue.
1 mark
(c)(i)(II) Sodium hydrogen carbonate ( $N a H C O_{3}$ ) and ammonium chloride ( $N H_{4} C l$ ) are formed:
$N a C l + H_{2} O + C O_{2} + N H_{3} \to N H_{4} C l + N a H C O_{3}$
1 mark
(c)(ii) A is bleaching powder ( $C a O C l_{2}$ ).
1 mark
(c)(ii) Equation: $C a (O H)_{2} + C l_{2} \to C a O C l_{2} + H_{2} O$
1 mark

Answer: (a) X = Cl₂, Y = H₂; (b) 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂; (c)(i) red litmus turns blue; NaHCO₃ and NH₄Cl formed; (c)(ii) A = bleaching powder, Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Where the marks are

X = chlorine (½ mark)
Y = hydrogen (½ mark)
Balanced chlor-alkali equation (1 mark; ½ deducted if unbalanced)
(c)(i)(I) Red litmus turns blue (1 mark)
(c)(i)(II) NaHCO₃ and NH₄Cl formed or equation (1 mark)
(c)(ii) Identifies bleaching powder (1 mark)
(c)(ii) 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 paper2026 board paper, set 31/2/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.

CBSE Class 10 Science 2026: question 28

Hints

Step-by-step solution

Revise the chapter

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