Problem 20 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

During depression of freezing point in a solution, the following are in equililbrium (a) liquid solvent, solid solvent (b) liquid solvent, solid solute (c) liquid solute, solid solute (d) liquid solute, solid solvent

Step-by-Step Solution

Answer

(a) Liquid solvent, solid solvent.

Step 1: Understanding Freezing Point Depression

Freezing point depression is a colligative property, which means it depends on the number of solute particles in a solution. When a non-volatile solute is added to a solvent, it lowers the freezing point of the solvent.

Step 2: Identifying Equilibrium Phases

During freezing point depression, the solution and the pure solvent are involved. The system in equilibrium consists of the liquid phase of the solvent and the solid phase of the solvent.

Step 3: Reviewing Answer Choices

We analyze the given options to see which one fits the description of phases in equilibrium during freezing point depression: (a) Liquid solvent, solid solvent (b) Liquid solvent, solid solute (c) Liquid solute, solid solute (d) Liquid solute, solid solvent

Step 4: Selecting the Correct Option

The correct equilibrium involves the liquid phase of the solvent and the solid phase of the solvent, which corresponds to option (a). This means the solvent is undergoing a phase change from liquid to solid.

Key Concepts

Freezing Point Depression

Freezing point depression is a fascinating colligative property. It refers to the lowering of a solvent’s freezing point when a non-volatile solute is dissolved in it. This effect occurs due to the disruption of the orderly crystal lattice formation by solute particles. In simpler terms, having more particles in the solvent makes it harder for the solution to become solid, thus lowering its freezing point.

Factors that influence the magnitude of freezing point depression include:

• Concentration of the solute: More particles mean more disruption and a greater depression.
• The specific solvent and solute: Different substances interact differently, impacting the degree of freezing point change.

Remember, it doesn't matter what kind of particles you add; what matters more is how many of them there are! This is because colligative properties are generally dependent on the number of solute particles, not their identity.

Phase Equilibrium

Phase equilibrium plays a key role during freezing point depression. When a solution is at its freezing point, the system reaches a state of balance between two phases: liquid and solid. Specifically, it involves the liquid form of the solvent and the solid form of the solvent in equilibrium.

Here’s what happens:

• As the solution starts to freeze, the solvent molecules in the liquid phase begin to arrange themselves into a solid structure.
• At equilibrium, the rate at which the liquid turns into solid is equal to the rate at which the solid turns back into liquid.

This delicate balance ensures that the temperature remains constant at the freezing point. Understanding this equilibrium is crucial to knowing why the solution stabilizes at a particular lower temperature compared to the pure solvent.

Solvent and Solute Interaction

The interaction between solvent and solute is fundamental to understanding the concept of freezing point depression. When a solute is added to a solvent, the particles of the solute interfere with the interactions between the molecules of the solvent.

This interference affects the formation of the solid phase by:

• Lowering the energy required for the solvent molecules to organize into a crystal lattice (the solid form).
• Increasing the degree of disorder or entropy in the solution.

The greater the number of solute particles present, the more significant the effect on the freezing point. Through these interactions, the solute essentially makes it more "difficult" for the solvent to freeze, requiring lower temperatures for solid formation. This interplay of forces exemplifies why the identity of the particles isn’t crucial in colligative properties — it's all about the interaction and quantity of particles involved.