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Amrita electrolysed… (Class 10 Science SQP 2025-26, Q27)

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ScienceCBSE Class 10 · Sample paper, session 2025-26 · 3 marks · Case-based

CBSE Class 10 Science sample paper 2025-26: question 27

From the official CBSE sample question paper for 2025-26, section B, question 27, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27 · 3 marks · Case-basedCompetency-basedChemical Reactions and Equations

Amrita electrolysed distilled water using the set-up shown in figure 1. She was expecting two gases to be evolved at the anode and cathode respectively. Suddenly, she realised that the bulb in the circuit did not glow when she used distilled water (figure 2). After this realization, she added a substance to the distilled water for electrolysis to take place. Answer the following questions based on the information given above:

  1. A

    Which gas was she expecting to be formed at the anode and which one at the cathode respectively?

    1 mark
  2. B

    Why did the bulb not glow when Amrita passed electricity through distilled water?

    1 mark
  3. C

    Which substance was added by Amrita to distilled water to get the expected result?

    1 mark

Figure in the paper: Figure 1: A plastic mug containing water, with two test tubes inverted over two graphite rods (electrodes) dipped in the water. The rod under the left tube is the anode and the rod under the right tube is the cathode. Both rods are connected by wires through a switch to a 6 V battery. Figure 2: A circuit with a battery, a bulb and a conducting wire, the circuit completed by dipping the two ends of the wire into a beaker of distilled water. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Water splits into two gases. Which one is released at the positive electrode?

Hint 2

What does a liquid need in order to conduct electricity?

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.

  1. A. In the electrolysis of water, oxygen is expected at the anode and hydrogen at the cathode. The volume of hydrogen is double that of oxygen: .

    1 mark

  2. B. Distilled water has almost no ions, so it is a very poor conductor of electricity. Too little current flows for the bulb to glow.

    1 mark

  3. C. She added a few drops of dilute sulphuric acid (or some common salt / any strong electrolyte). This provides ions so that current flows and electrolysis takes place.

    1 mark

  4. For visually impaired students (in place of A–C, 1 mark each): A. is a redox reaction (ZnO is reduced, C is oxidised). B. is a decomposition reaction and is endothermic (needs heat). C. + heat is a combination reaction and is exothermic.

Answer: A. Oxygen at anode, hydrogen at cathode. B. Distilled water is a poor conductor (no ions). C. Few drops of dilute H₂SO₄ (or NaCl). VI: redox; decomposition (endothermic); combination (exothermic).

Where the marks are

  • O₂ at anode and H₂ at cathode (1 mark)
  • Distilled water is a poor conductor / lacks ions (1 mark)
  • Adds dilute H₂SO₄ or NaCl or another electrolyte (1 mark)
  • VI: redox; decomposition/endothermic; combination/exothermic (1 mark each)

Revise the chapter

Practise this chapterBoard-style questions from Chemical Reactions and Equations, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: 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.