JEE Chemistry 2024 Question with Solution

Given: Naphthalene is added in a small quantity to benzene.

Find: What happens to the freezing point of benzene.

This is a case of freezing point depression, which is a colligative property. When a non-volatile solute is dissolved in a solvent, the freezing point of the solvent decreases.

The relevant relation is:

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

where $\Delta T_f$ is the depression in freezing point, $i$ is the van 't Hoff factor, $K_f$ is the cryoscopic constant, and $m$ is the molality.

When naphthalene is added to benzene, naphthalene acts as the solute and benzene acts as the solvent. Therefore, the freezing point of benzene is lowered.

Therefore, the correct option is D and the freezing point decreases.