Identify the type of solution

Naphthalene is a non-volatile solute when dissolved in benzene. Adding a small amount of solute to a pure solvent affects its colligative properties.

Relevant principle: Depression in freezing point

For a dilute solution, the freezing point decreases according to:

$\Delta T_f = K_f m$

where:

• $\Delta T_f$ = lowering of freezing point
• $K_f$ = molal depression constant of the solvent
• $m$ = molality of the solution

Apply to benzene + naphthalene

Since naphthalene dissolves in benzene, the solution formed has a freezing point lower than that of pure benzene.

Hence, the freezing point of benzene decreases.

Check options

• A: Increases $\rightarrow$ Incorrect
• B: Decreases $\rightarrow$ Correct
• C: Remains unchanged $\rightarrow$ Incorrect
• D: First decreases and then increases $\rightarrow$ Incorrect

Final answer

The freezing point of benzene decreases when a small quantity of naphthalene is added.