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A student performs the… (Class 10 Science 2025, Q21)

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ScienceCBSE Class 10 · Board exam 2025, set 31/1/1 · 2 marks · Very short answer

CBSE Class 10 Science 2025: question 21

Set in the CBSE Class 10 board exam of 2025, paper 31/1/1, section B, question 21, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21 · 2 marks · Very short answerChemical Reactions and Equations

A student performs the following experiment in his school laboratory. List two observations to justify that in this experiment a chemical change has taken place.

Figure in the paper: A conical flask containing dilute sulphuric acid with zinc (Zn) granules at the bottom, fitted with a cork through which a delivery tube/rod passes. See it in the official paper.

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

Hints

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

Hint 1

Think about what you would see around the zinc granules.

Hint 2

Touch the flask: what do you feel?

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. Reaction:

  2. Observation 1: Bubbles of a gas (hydrogen) form on the zinc granules, which shows that a gas is evolved.

    1 mark

  3. Observation 2: The conical flask becomes warm, which shows a change (rise) in temperature.

    1 mark

Answer: Evolution of gas (H₂ bubbles) and rise in temperature

Where the marks are

  • States that a gas is evolved (1 mark)
  • States that the temperature changes or rises (1 mark)

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 paper2025 board paper, set 31/1/1: 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.