What happens to freezing point of benzene when small quantity of naphthalene is added to benzene?

We need to determine what happens to the freezing point of benzene when a small quantity of naphthalene is added.

Key Concept: Depression in Freezing Point (Colligative Property) When a non-volatile solute is dissolved in a solvent, the freezing point of the solution decreases. This is known as the depression of freezing point, a colligative property.

The depression in freezing point is given by:

$$\Delta T_f = K_f \cdot m$$

where $$K_f$$ is the cryoscopic constant of the solvent and $$m$$ is the molality of the solution.

Naphthalene is a non-volatile, non-electrolyte solute. When it is dissolved in benzene (the solvent), it forms a solution and lowers the vapour pressure of benzene. This causes the solid-liquid equilibrium to shift, requiring a lower temperature for freezing to occur. Since $$\Delta T_f = K_f \cdot m > 0$$, the freezing point of the solution is lower than that of pure benzene.

Therefore, the freezing point of benzene decreases when naphthalene is added.

Answer: Option 4 - Decreases