If common salt is dissolved in water, the vapour pressure of the solution will

Options

A. Increase
B. Decrease
C. Remain unchanged
D. Can not be predicted

Answer

To solve the question regarding the effect of dissolving common salt (NaCl) in water on the vapor pressure of the solution, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Solute and Solvent**:
- The solute is common salt (NaCl) and the solvent is water (H₂O).

2. **Understand the Nature of the Solute**:
- Common salt (NaCl) is a non-volatile solute. This means it does not evaporate and does not contribute to the vapor pressure of the solution.

3. **Recall the Concept of Vapor Pressure**:
- Vapor pressure is the pressure exerted by the vapor of a liquid in equilibrium with its liquid phase. When a non-volatile solute is added to a solvent, it affects the vapor pressure of the solvent.

4. **Apply Raoult's Law**:
- According to Raoult's Law, the vapor pressure of a solvent in a solution (P_solution) is directly proportional to the mole fraction of the solvent (X_solvent):
\[ P_{\text{solution}} = X_{\text{solvent}} \times P^0_{\text{solvent}} \]
- Where \( P^0_{\text{solvent}} \) is the vapor pressure of the pure solvent.

5. **Determine the Effect of Adding NaCl**:
- When NaCl is dissolved in water, it dissociates into Na⁺ and Cl⁻ ions. This increases the number of solute particles in the solution.
- The presence of these solute particles reduces the mole fraction of the solvent (water), which in turn decreases the vapor pressure of the solution.

6. **Conclusion**:
- Therefore, when common salt is dissolved in water, the vapor pressure of the solution will **decrease**.

### Final Answer:
The vapor pressure of the solution will decrease when common salt is dissolved in water.

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