What happens to freezing point of benzene when small quantity of napthalene is added to benzene?
- Increases
- Remains unchanged
- First decreases and then increases
- Decreases
The Correct Option is D
Solution and Explanation
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
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