Q1. [5] medium exam-ready
What is water of crystallisation? A student heats blue crystals of copper sulphate in a boiling tube and then adds a few drops of water to the residue. State the observations made at each step and explain what they indicate. Write the chemical formula of hydrated copper sulphate and of gypsum.
Generated by claude-sonnet-4-6 · 2026-06-15 06:42 · grounding rag
Model Answer
Water of crystallisation is the fixed number of water molecules chemically bonded in the crystal structure of a salt. For example, hydrated copper sulphate contains 5 water molecules per formula unit.
Observations and explanations:
- On heating — Blue crystals of copper sulphate turn white/grey. This indicates that the water of crystallisation has been driven out, leaving anhydrous copper sulphate (white residue).
- On adding water to the white residue — The white residue turns blue again and the test tube becomes warm. This indicates that the water of crystallisation has been restored and the change is reversible (exothermic rehydration).
Chemical formulae:
- Hydrated copper sulphate (Blue vitriol): $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$
- Gypsum: $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$
Source: Chapter 1, Section 1.2.2
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Explanation
- The question has four parts: define water of crystallisation, observation on heating, observation on adding water, and two formulae — cover all four for full marks.
- Examiners specifically look for the colour change (blue → white → blue) and the word "reversible" or the idea that water of crystallisation can be restored.
- The warmth felt on adding water (exothermic) is a good scoring point.
- Do not confuse gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) with Plaster of Paris ($\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$).