Which one of the statements given below-concerning properties of solutions, describes a colligative effect?

Vapour pressure of pure benzene decreases by the addition of naphthalene

The correct answer is option 3. Vapour pressure of pure benzene decreases by the addition of naphthalene.

Let us delve deeper into the concept of colligative properties and why the given statement is an example of one.

Colligative properties depend on the number of solute particles present in a solution, not on the nature of the solute. There are four main colligative properties:

1. Elevation of Boiling Point: Adding a solute to a solvent increases the boiling point of the solution compared to the pure solvent. This occurs because the solute particles interfere with the ability of solvent molecules to escape into the vapor phase, requiring more heat to reach the boiling point.

2. Depression in Freezing Point : Adding a solute to a solvent lowers the freezing point of the solution. This is because the solute particles disrupt the formation of the solid structure, thus lowering the temperature at which the solution freezes.

3. Lowering of Vapor Pressure: The presence of a non-volatile solute lowers the vapor pressure of the solvent. This occurs because the solute particles occupy space at the surface, reducing the number of solvent molecules that can escape into the vapor phase.

4. Osmotic Pressure: The pressure required to stop the flow of solvent through a semipermeable membrane into a solution with a higher concentration of solute. This pressure depends on the number of solute particles in the solution.

Explanation of the Statement: Vapor pressure of pure benzene decreases by the addition of naphthalene

Vapor Pressure: This is the pressure exerted by the vapor of a liquid in equilibrium with its liquid phase at a given temperature.

Addition of Naphthalene: Naphthalene is a non-volatile solid, meaning it does not easily evaporate. When added to benzene, it does not contribute to the vapor pressure directly but rather affects the vapor pressure of benzene.

Why Does the Vapor Pressure Decrease?

When naphthalene is added to benzene:

1. Disruption of Surface: Naphthalene particles disrupt the surface of the benzene, reducing the number of benzene molecules that can escape into the vapor phase. This decreases the vapor pressure of benzene.

2. Raoult’s Law: According to Raoult’s Law, the addition of a non-volatile solute to a solvent lowers the solvent's vapor pressure proportionally to the concentration of the solute. The more solute particles present, the lower the vapor pressure of the solvent.

Conclusion: The decrease in vapor pressure of benzene due to the addition of naphthalene is a classic example of a colligative property. This decrease occurs because the naphthalene particles reduce the number of benzene molecules that can vaporize, which directly affects the vapor pressure of the solution.