Which is not a colligative property?

Options

A. Relative lowering of vapour pressure
B. Freezing point
C. Osmotic pressure
D. Elevation of boiling point

Answer

To solve the question "Which is not a colligative property?", we need to analyze the provided options and determine which one does not fit the definition of colligative properties.

### Step-by-Step Solution:

1. **Understand Colligative Properties**:
- Colligative properties are properties that depend on the number of solute particles in a solution, not on the nature or type of solute.
- Common colligative properties include relative lowering of vapor pressure, elevation of boiling point, depression of freezing point, and osmotic pressure.

2. **Analyze Each Option**:
- **Relative Lowering of Vapor Pressure**:
- This property is defined as the decrease in vapor pressure of a solvent when a solute is added. It is directly proportional to the mole fraction of the solute, which depends on the number of solute particles. Therefore, it is a colligative property.

- **Freezing Point**:
- The freezing point itself is not a colligative property. However, the depression of freezing point (ΔTf) is a colligative property, as it depends on the number of solute particles. Thus, "freezing point" alone does not qualify as a colligative property.

- **Osmotic Pressure**:
- Osmotic pressure (π) is given by the formula π = CRT, where C is the concentration of the solution. This pressure depends on the number of solute particles, making it a colligative property.

- **Elevation of Boiling Point**:
- The elevation of boiling point (ΔTb) is also a colligative property, as it depends on the number of solute particles in the solution. It is calculated using the formula ΔTb = Kb * molality.

3. **Identify the Correct Answer**:
- Among the options analyzed, "freezing point" is the only term that does not represent a colligative property on its own. The correct answer is therefore **freezing point**.

### Conclusion:
The option that is not a colligative property is **freezing point**.