The plaster of paris is hardened by

Options

A. Liberating `CO_(2)`
B. Giving out water
C. Combining with water
D. Changing into `CaCO_(3)`

Answer

To solve the question "The plaster of Paris is hardened by," we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Composition of Plaster of Paris**:
- Plaster of Paris is chemically known as calcium sulfate hemihydrate, which has the formula \( \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} \).

2. **Understanding the Setting Process**:
- When plaster of Paris is mixed with water, it undergoes a chemical reaction. The hemihydrate form reacts with water to form the dihydrate form of calcium sulfate.

3. **Chemical Reaction**:
- The reaction can be represented as:
\[ \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} + \text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2 \text{H}_2\text{O} \]
- This shows that one mole of plaster of Paris reacts with one mole of water to form calcium sulfate dihydrate.

4. **Result of the Reaction**:
- The product formed, calcium sulfate dihydrate (\( \text{CaSO}_4 \cdot 2 \text{H}_2\text{O} \)), is what causes the plaster to harden.

5. **Conclusion**:
- Therefore, we conclude that plaster of Paris is hardened by combining with water.

### Final Answer:
The plaster of Paris is hardened by **combining with water**.