Q1. [1] medium thorough-understanding
Which of the following best explains why Plaster of Paris must be stored in a moisture-proof container?
(A) Moisture causes Plaster of Paris to decompose into calcium oxide and water.
(B) Moisture reacts with Plaster of Paris to convert it back to gypsum, making it hard and unusable.
(C) Moisture dissolves Plaster of Paris, washing it away.
(D) Moisture causes Plaster of Paris to lose its water of crystallisation.
- A Moisture causes Plaster of Paris to decompose into calcium oxide and water.
- B Moisture reacts with Plaster of Paris to convert it back to gypsum, making it hard and unusable.
- C Moisture dissolves Plaster of Paris, washing it away.
- D Moisture causes Plaster of Paris to lose its water of crystallisation.
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
(B) Moisture reacts with Plaster of Paris to convert it back to gypsum, making it hard and unusable.
Explanation
Plaster of Paris is calcium sulphate hemihydrate (CaSO₄·½H₂O). When it absorbs moisture, it rehydrates to form gypsum (CaSO₄·2H₂O), setting hard — making it useless for further application. Options A, C, and D describe incorrect reactions. Examiners expect students to recall the reversible relationship between gypsum and Plaster of Paris.