Write the chemical equation for the reaction of Plaster of Paris with water.
Generated by claude-sonnet-4-6 · 2026-06-17 12:36 · grounding rag
Model Answer
Plaster of Paris (calcium sulphate hemihydrate) reacts with water to form gypsum (calcium sulphate dihydrate). The chemical equation is:
$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}(s) + \frac{3}{2}\text{H}_2\text{O}(l) \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}(s)$$
$$\text{(Plaster of Paris)} \hspace{3cm} \text{(Gypsum)}$$
This reaction is exothermic and the mixture sets into a hard solid mass.
Source: Chapter 2, Acids, Bases and Salts
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Explanation
- The examiner expects the correct chemical formula of Plaster of Paris ($\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$) and gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) — these formulas fetch the marks.
- Mention that the product is gypsum and that the mixture sets hard — this shows understanding and is often expected for full marks.
- This question is directly linked to Exercise Q13 of Chapter 2: "Plaster of Paris should be stored in a moisture-proof container" — the same reaction explains why (it absorbs moisture and sets hard, becoming useless).