Science — AI-generated practice question

Plaster of Paris (POP) is a white powder with the chemical formula $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$ (calcium sulphate hemihydrate). It is used in making casts for fractured bones, toys, and decorative materials.

Preparation from Gypsum:
Gypsum is $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$ (calcium sulphate dihydrate). When gypsum is heated at 373 K (100°C), it loses three-fourths of its water of crystallisation to form Plaster of Paris:

$$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O}$$

On mixing with water, POP sets into a hard solid (gypsum) again.

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• 1 mark for definition/formula of POP.
• 1 mark for the correct condition (heating at 373 K) and starting material (gypsum with formula).
• 1 mark for the balanced chemical equation.
• Examiners expect the equation, both formulas, and the temperature 373 K — these are the most common marks lost. The reverse reaction (setting of POP) is a bonus line but not mandatory.