Science — AI-generated practice question

(a) When gypsum is heated at 373 K, it loses water of crystallisation to form Plaster of Paris:

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

(Gypsum) $\hspace{3.5cm}$ (Plaster of Paris)

(b) Plaster of Paris reacts with water to convert back to gypsum:

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

(c) When Plaster of Paris is mixed with water, it sets into a hard, solid mass (gypsum) within a few minutes. This property makes it useful for setting fractured bones — it holds the broken bone firmly in the correct position while the bone heals, and the cast can be easily removed later.

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• (a): The key detail is heating at 373 K and the formula of Plaster of Paris as CaSO₄·½H₂O. Many students incorrectly write CaSO₄ (anhydrous) — include the ½H₂O.
• (b): This is simply the reverse reaction — addition of water reconverts POP to gypsum.
• (c): Examiners want two points: (i) POP sets hard on mixing with water and (ii) this holds the bone in place during healing. Don't just say "it hardens" — link it to the medical use.