State what is meant by 'water of crystallisation'. Give one example of a salt that contains water of crystall…

Question

State what is meant by 'water of crystallisation'. Give one example of a salt that contains water of crystallisation, stating its chemical formula, and describe what you would observe on strongly heating a few crystals of this salt.

Accepted Answer

## Model Answer **Water of crystallisation** is the fixed number of water molecules present in one formula unit of a salt. **Example:** Copper sulphate — CuSO₄·5H₂O **Observation on strong heating:** The blue crystals turn white (as water of crystallisation is removed), and water droplets appear on the cooler parts of the boiling tube. ## Explanation The examiner expects: (1) a correct definition, (2) a named example with correct formula, and (3) two observations — colour change (blue → white) and water droplets. Copper sulphate is the textbook example used in Activity 2.15. Washing soda (Na₂CO₃·10H₂O) is also acceptable as an example. Don't forget the formula — it earns a mark by itself. Source: Chapter 2, Section 2.4.4

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