Solute when dissolved in water:

Options

A. increases the vapour pressure of water
B. decreases the boiling point of water
C. decreases the freezing point of water
D. all of the above

Answer

To solve the question regarding the effect of a solute when dissolved in water, we will analyze the options provided and apply the relevant colligative properties of solutions.

### Step-by-Step Solution:

1. **Understanding the Problem:**
- We need to determine how the addition of a solute affects the properties of water, specifically its vapor pressure, boiling point, and freezing point.

2. **Vapor Pressure:**
- According to Raoult's Law, the vapor pressure of a solvent (water in this case) is given by:
\[ P = P^0 \cdot X_a \]
where \( P^0 \) is the vapor pressure of pure solvent and \( X_a \) is the mole fraction of the solvent.
- When a solute is added, the mole fraction of the solvent decreases, leading to a decrease in vapor pressure.
- Therefore, the statement "increase the vapor pressure of water" is incorrect; the vapor pressure actually **decreases**.

3. **Boiling Point Elevation:**
- The boiling point elevation (\( \Delta T_b \)) is given by:
\[ \Delta T_b = K_b \cdot m \]
where \( K_b \) is the ebullioscopic constant and \( m \) is the molality of the solution.
- Since the number of moles of solute increases, the molality increases, leading to an increase in the boiling point of water.
- Therefore, the statement "decrease the boiling point of water" is incorrect; the boiling point actually **increases**.

4. **Freezing Point Depression:**
- The freezing point depression (\( \Delta T_f \)) is given by:
\[ \Delta T_f = K_f \cdot m \]
where \( K_f \) is the cryoscopic constant and \( m \) is the molality of the solution.
- As the molality increases with the addition of solute, the freezing point decreases.
- Therefore, the statement "decrease the freezing point of water" is correct; the freezing point actually **decreases**.

5. **Conclusion:**
- Based on the analysis of the effects of a solute on the properties of water:
- The vapor pressure of water **decreases**.
- The boiling point of water **increases**.
- The freezing point of water **decreases**.
- Thus, the correct answer is that the solute decreases the freezing point of water.

### Final Answer:
The solute when dissolved in water decreases the freezing point of water.

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