Plaster of Paris is:
- Ca(OH)₂
- CaSO₄·½H₂O
- CaCO₃
- CaO
Show answer and solution
Correct answer: B. CaSO₄·½H₂O
### Step-by-Step Solution
**1. Definition and Composition:**
Plaster of Paris (PoP) is a quick-setting gypsum plaster consisting of a fine white powder (calcium sulphate hemihydrate). Its chemical name is **Calcium Sulphate Hemihydrate**.
**2. Chemical Formula:**
The term 'hemihydrate' refers to the presence of half a molecule of water of crystallization for every molecule of calcium sulphate. Therefore, the formula is written as:
$$CaSO_4 \cdot \frac{1}{2}H_2O$$
**3. Formation from Gypsum:**
It is prepared by heating Gypsum ($CaSO_4 \cdot 2H_2O$) to $373\text{ K}$ ($100^\circ\text{C}$). At this temperature, gypsum loses water molecules to become Plaster of Paris:
$$CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K}} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O$$
**4. Analysis of Options:**
* $Ca(OH)_2$ is Calcium Hydroxide (Slaked Lime).
* $CaSO_4 \cdot \frac{1}{2}H_2O$ is Calcium Sulphate Hemihydrate (Plaster of Paris).
* $CaCO_3$ is Calcium Carbonate (Limestone).
* $CaO$ is Calcium Oxide (Quicklime).
**Conclusion:**
The correct formula for Plaster of Paris is **$CaSO_4 \cdot \frac{1}{2}H_2O$**.
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