Explain why Plaster of Paris must be stored in a moisture-proof container. Write the chemical equation to support your answer.
Generated by claude-sonnet-4-6 · 2026-06-17 12:38 · grounding rag
Model Answer
Plaster of Paris (CaSO₄·½H₂O) must be stored in a moisture-proof container because it readily reacts with atmospheric moisture (water) and gets converted back into gypsum (CaSO₄·2H₂O). Once this happens, the Plaster of Paris loses its ability to set hard when water is added later, making it useless.
Chemical Equation:
$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$
$$\text{(Plaster of Paris)} \hspace{1.5cm} \text{(Gypsum)}$$
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Explanation
- This question tests knowledge of Plaster of Paris from the Acids, Bases and Salts chapter (Chapter 2).
- The key point examiners look for: why moisture is harmful (it converts POP → Gypsum, causing it to set prematurely and lose utility).
- Always write the balanced chemical equation with correct formula — the ½H₂O (hemihydrate) notation is essential for full marks.
- Note: The source passages provided did not cover this topic; the answer is drawn from the standard CBSE Class 10 Chemistry syllabus (Chapter 2 — Acids, Bases and Salts).