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When blue copper sulphate crystals are gently heated until they turn colourless, what has happened?
AThe salt has gained water and become hydrated
BThe salt has lost its water of crystallization
CThe copper has been oxidised to copper oxide
DThe salt has dissolved into a saturated solution
Answer & Solution
Correct answer: B. The salt has lost its water of crystallization
1. Blue copper sulphate is the hydrated salt CuSO4.5H2O; its blue colour is due to water of crystallization.
2. On gentle heating it loses its five water molecules and becomes colourless anhydrous copper sulphate.
3. It does not gain water; that is the reverse change that turns it blue again, so that option is wrong.
4. Therefore heating removes the water of crystallization.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 9 Solutions "Copper sulphate pentahydrate", p.137_
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