Generated by claude-sonnet-4-6 · 2026-06-14 10:29 · grounding rag
Model Answer
Copper sulphate crystals are blue because they contain water of crystallisation (CuSO₄·5H₂O). On heating, they lose this water of crystallisation and turn white, forming anhydrous copper sulphate (CuSO₄). This is a decomposition reaction.
$$\text{CuSO}_4 \cdot 5\text{H}_2\text{O} \xrightarrow{\text{Heat}} \text{CuSO}_4 + 5\text{H}_2\text{O}$$
Source: Chapter 1, Section 1.2.2 Decomposition Reaction
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Explanation
- The key concept here is water of crystallisation — the fixed number of water molecules present in one formula unit of a crystallised salt.
- Examiners expect you to name both forms: CuSO₄·5H₂O (blue, hydrated) and CuSO₄ (white, anhydrous).
- Mentioning it is a decomposition reaction (thermal decomposition) adds marks.
- The equation is important — write it correctly with the dot notation for water of crystallisation.