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.
Generated by claude-sonnet-4-6 · 2026-06-14 10:29 · grounding rag
Model Answer
(a) 'X' is ferrous sulphate (FeSO₄·7H₂O) and 'Y' is ferric oxide (Fe₂O₃).
The reaction is:
$$2\text{FeSO}_4 \xrightarrow{\text{Heat}} \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$$
(b) On heating, the green colour of ferrous sulphate crystals first changes to white (on losing water), and then turns reddish-brown (colour of Fe₂O₃) on further heating.
Source: Chapter 1, Section 1.2.2 – Decomposition Reaction
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Explanation
- Examiners expect you to name both substances with their chemical formulas and include the balanced equation (it adds precision and earns marks).
- For part (b), mention two colour changes: green → white (loss of water) → reddish-brown (Fe₂O₃ formed). Mentioning only one change may cost you a mark.
- The "characteristic odour" in the question refers to SO₂/SO₃ gases — this confirms it is ferrous sulphate decomposing.