When ferrous sulphate crystals are heated strongly, three products are formed. What are they, and how can you tell — from observation alone — that a chemical change has occurred rather than just a physical change?
Generated by claude-sonnet-4-6 · 2026-06-26 00:54 · grounding rag
Model Answer
When ferrous sulphate (FeSO₄) crystals are heated strongly, they decompose to form three products:
- Ferric oxide (Fe₂O₃) — a reddish-brown solid
- Sulphur dioxide (SO₂) — a gas
- Sulphur trioxide (SO₃) — a gas
$$2\text{FeSO}_4 \xrightarrow{\Delta} \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$$
Evidence of chemical change (from observation alone):
- The colour changes from green (FeSO₄ crystals) to reddish-brown (Fe₂O₃).
- A characteristic odour of burning sulphur is produced (due to SO₂/SO₃ gases).
These observations — colour change and gas evolution with odour — confirm a chemical change, not merely a physical one.
Source: Chapter 1, Section 1.2.2 — Decomposition Reaction
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Explanation
- Examiners expect you to name all three products with correct formulae — each carries marks.
- The equation (optional but good) shows it's a decomposition/thermal decomposition reaction.
- For "observation alone," stick to what you see (colour change: green → reddish-brown) and smell (sulphur odour) — do not mention formulae or theory here.
- A physical change is reversible and produces no new substance; colour change + new gas with odour proves new substances have formed → chemical change.