Question

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

• A. Increases
• B. Remains unchanged
• C. First decreases and then increases
• D. Decreases

Answer 1

The Correct Option is D

The question focuses on the effect of adding a small quantity of naphthalene to benzene on its freezing point. This concept is related to colligative properties in chemistry, specifically freezing point depression. Here’s a detailed explanation:

Understanding Freezing Point Depression

The freezing point of a pure substance is the temperature at which it changes from a liquid to a solid. When a solute is dissolved in a solvent, the freezing point of the solvent decreases. This phenomenon is known as freezing point depression, a colligative property which depends on the number of solute particles in a solvent, not the nature of the solute itself.

Key Formula

The depression in freezing point can be calculated using the formula:

\(\Delta T_f = i \cdot K_f \cdot m\)

• \(\Delta T_f\) is the change in freezing point.
• \(i\) is the van 't Hoff factor (number of particles the solute splits into).
• \(K_f\) is the cryoscopic constant of the solvent (benzene in this case).
• \(m\) is the molality of the solution.

Application to the Given Problem

When naphthalene is added to benzene:

• Naphthalene acts as a solute and benzene as a solvent.
• The addition of naphthalene particles to benzene results in freezing point depression according to the formula above.

Conclusion

Based on the concept of freezing point depression, when a small quantity of naphthalene is added to benzene, the freezing point of benzene decreases.

Correct Answer: Decreases