Problem 42 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In the depression of freezing point experiment, it is found that the (a) vapour pressure of the solution is less than that of pure solvent (b) vapour pressure of the solution is more than that of pure solvent (c) only solute molecules solidify at the freezing point (d) only solvent molecules solidify at the freezing

Step-by-Step Solution

Answer

The correct options are (a) and (d).

Step 1: Understanding Freezing Point Depression

Freezing point depression occurs when a solute is added to a solvent, resulting in a lower freezing point for the solution compared to the pure solvent. This is because the solute particles disrupt the formation of a solid lattice of the solvent molecules, requiring a lower temperature to achieve solidification.

Step 2: Analyzing Vapor Pressure Changes

Adding a non-volatile solute to a solvent results in a decrease in the vapor pressure of the solution compared to the pure solvent. This occurs because the solute particles take up space at the liquid surface, reducing the number of solvent molecules that can escape into the vapor phase.

Step 3: Considering Solidification Process

At the freezing point, only the solvent molecules typically solidify because the solute usually remains dissolved in the unfrozen solution. The presence of solute lowers the chemical potential of the solvent, which shifts the equilibrium between the liquid and solid phases of the solvent.

Step 4: Correct Option Identification

Based on the understanding of freezing point depression: (a) is correct as the vapor pressure of the solution is less because of the solute, and (d) is correct as in typical cases only solvent molecules solidify.

Key Concepts

Colligative Properties

Colligative properties are unique because they depend solely on the number of solute particles in a solution, not the nature of the particles themselves. This means that whether you're dissolving sugar or salt in water, the effect will be the same if the number of dissolved particles is identical. This fascinating aspect makes colligative properties a great topic for understanding how different solutions behave.

One of the key properties is **freezing point depression**, which occurs when the freezing point of the solvent lowers after a solute is added. Why does this happen?

• The solute molecules disrupt the formation of a regular solid structure of the solvent, making it harder for the solvent to freeze.
• This results in needing a lower temperature to achieve solidification.

Other colligative properties include boiling point elevation, vapor pressure lowering, and osmotic pressure. The common thread is the number of solute particles influencing the solvent's physical properties.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid form in a closed system. This concept is vital to understand how solutions change when a solute is added.

When a non-volatile solute is added to a solvent, the vapor pressure of the overall solution decreases compared to the pure solvent. But why?

• The solute particles occupy space at the surface of the liquid, reducing the area available for solvent molecules to escape into the vapor phase.
• As fewer solvent molecules can escape, the vapor pressure decreases.

This change plays a significant role in colligative properties, linking closely with changes in boiling and freezing points of a solution. Lower vapor pressure means a solution has a lower tendency to evaporate, which can also affect how it freezes and boils.

Solvent-Solute Interaction

Solvent-solute interactions are at the heart of how colligative properties manifest. When a solute is dissolved in a solvent, it interacts with the solvent molecules, and this interaction significantly alters the physical properties of the solvent.

In the case of freezing point depression:

• The solute disrupts the orderly arrangement necessary for the solvent molecules to form a solid.
• This disruption requires a lower temperature to reach the solid phase, explaining why the freezing point is lowered.

During this process, it's important to note that usually only the solvent molecules solidify because the presence of solute lowers the solvent's chemical potential, which changes the solid-liquid equilibrium. These interactions are not just limited to freezing but extend to boiling and other physical changes within a solution. Understanding these interactions is fundamental for studying solution chemistry.