Which of the following is not a colligative property?

Options

A. Depression in freezing point
B. Elevation in boiling point
C. Osmotic pressure
D. Modification of refractive index

Answer

To determine which of the following is not a colligative property, we first need to understand what colligative properties are. Colligative properties depend on the number of solute particles in a solution, rather than the identity of the solute itself. The main colligative properties include:

1. **Relative lowering of vapor pressure**: This property is defined by the formula:
\[ \frac{P^0_a - P_a}{P^0_a} = X_b \]
where \(P^0_a\) is the vapor pressure of the pure solvent, \(P_a\) is the vapor pressure of the solution, and \(X_b\) is the mole fraction of the solute. This property depends on the number of moles of solute.

2. **Depression of freezing point**: This property can be expressed as:
\[ \Delta T_f = K_f \cdot m \]
where \(\Delta T_f\) is the depression in freezing point, \(K_f\) is the molal freezing point depression constant, and \(m\) is the molality of the solution. This also depends on the number of moles of solute.

3. **Elevation of boiling point**: This property is given by:
\[ \Delta T_b = K_b \cdot m \]
where \(\Delta T_b\) is the elevation in boiling point, \(K_b\) is the molal boiling point elevation constant, and \(m\) is the molality. Like the previous properties, this depends on the number of moles of solute.

4. **Osmotic pressure**: This is defined by the formula:
\[ \pi = cRT \]
where \(\pi\) is the osmotic pressure, \(c\) is the concentration of the solute, \(R\) is the gas constant, and \(T\) is the temperature. Again, this property depends on the number of solute particles.

Now, we need to identify which of the options provided is not a colligative property. The properties listed above (relative lowering of vapor pressure, depression of freezing point, elevation of boiling point, and osmotic pressure) are all colligative properties. However, the refractive index is not a colligative property as it does not depend solely on the number of solute particles but rather on the nature of the solute and solvent.

**Final Answer**: The property that is not a colligative property is the **refractive index**.