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A crystalline substance of… (Class 10 Science 2025, Q21)

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ScienceCBSE Class 10 · Board exam 2025, set 31/5/1 · 2 marks · Very short answer

CBSE Class 10 Science 2025: question 21

Set in the CBSE Class 10 board exam of 2025, paper 31/5/1, 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 answerAcids, Bases and Salts

A crystalline substance of green colour 'X' emits gases of characteristic odour when heated over a flame. It first loses water and changes colour. On further heating, it decomposes and produces a solid compound Y.

  1. (a)

    Identify 'X' and 'Y'.

    1 mark
  2. (b)

    State the change in colour observed when 'X' is heated.

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/5/1.

Hints

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

Hint 1

Which green crystalline salt in NCERT gives off gases smelling of burning sulphur when heated?

Hint 2

It contains 7 molecules of water of crystallisation; think about what is left after SO₂ and SO₃ escape.

Hint 3

Losing water changes the colour first; the iron oxide formed at the end is reddish-brown.

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) The green crystals that lose water of crystallisation and then decompose with a smell of burning sulphur are ferrous sulphate crystals: **X = ** (ferrous sulphate heptahydrate).

    ½ mark

  2. (a) On strong heating: . The solid left is **Y = ** (ferric oxide).

    ½ mark

  3. (b) The green crystals first turn white as they become anhydrous . On further heating the white solid turns reddish-brown as forms.

    1 mark

Answer: (a) X = FeSO₄·7H₂O (ferrous sulphate crystals), Y = Fe₂O₃ (ferric oxide); (b) green → white → reddish-brown

Where the marks are

  • Identifies X as ferrous sulphate crystals, FeSO₄·7H₂O (½ mark)
  • Identifies Y as ferric oxide, Fe₂O₃ (½ mark)
  • States the colour change green to white/brown (1 mark)

Revise the chapter

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 31/5/1: 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.