Q1. [3] medium initial-understanding
A white powder hardens when mixed with water and is used in making casts for broken bones.
(i) What is this substance? Write its chemical formula.
(ii) Name the compound it is prepared from, and state what happens when it is mixed with water.
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
(i) The substance is Plaster of Paris.
Chemical formula: $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$
(ii) It is prepared from Gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$). When Plaster of Paris is mixed with water, it rehydrates and sets into a hard solid mass (gypsum is reformed). This property makes it useful for making casts for broken bones.
$$\text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O} + \tfrac{3}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$
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Explanation
- Examiners expect the name Plaster of Paris with its correct formula (the ½ water of crystallisation is essential — a common error is writing 1H₂O).
- For part (ii), naming Gypsum as the parent compound earns 1 mark; stating that it sets hard / reverts to gypsum on mixing with water earns the remaining mark.
- The equation is a bonus/good practice but the key points are the name, formula, parent compound, and the setting reaction.