Write the common name and the chemical name of the compound CaSO₄·½H₂O. Write the method of its preparation. Give chemical equation for the reaction, when water reacts with CaSO₄·½H₂O.
Generated by claude-sonnet-4-6 · 2026-06-15 06:44 · grounding rag
Model Answer
Common name: Plaster of Paris
Chemical name: Calcium sulphate hemihydrate
Preparation: Plaster of Paris is prepared by heating gypsum (CaSO₄·2H₂O) at 373 K (100°C).
$$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O}$$
Reaction with water:
When water is added to Plaster of Paris, it sets into a hard solid mass — gypsum.
$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$
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Explanation
- Examiners expect all three parts: name, preparation with equation, and the reverse reaction equation.
- Note that Plaster of Paris sets by recombining with water to reform gypsum — this is the reverse of its preparation, so knowing both equations together is essential.
- The formula CaSO₄·½H₂O (or CaSO₄·¼H₂O in some books) — NCERT Class 10 Science uses the ½H₂O form; always match your textbook edition.
- The temperature 373 K is a key detail examiners look for.