How is plaster of Paris prepared ? Why is plaster of Paris stored in an airtight container ?

Explanation

### Step-by-Step Solution:

**Step 1: Preparation of Plaster of Paris**
- Plaster of Paris is prepared by heating gypsum (CaSO4·2H2O) at a temperature of about 373 Kelvin (approximately 100°C).
- The chemical reaction that occurs is as follows:
\[ \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{\text{heat}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \text{H}_2\text{O (vapor)} \]
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