Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

During depression in freezing point in a solution, the following are in equilibrium (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: Understand Freezing Point Depression

Freezing point depression is a colligative property observed when a solute is dissolved in a solvent, leading to a solution that freezes at a lower temperature than the pure solvent. The equilibrium involved is between the liquid and solid phases.

Step 2: Identify Phases in Equilibrium

During freezing point depression, the equilibrium is between the liquid phase of the solvent and the solid phase of the solvent. This means we are considering the solvent freezing and not the solute.

Step 3: Match Options with Equilibrium

Given the nature of freezing point depression, the liquid and solid phases of the solvent are involved in the equilibrium. Therefore, option (a), which describes 'liquid solvent, solid solvent', correctly represents this equilibrium state.

Key Concepts

Colligative Properties

Colligative properties are unique characteristics of solutions that depend solely on the number of solute particles present, not their identity. This means that when any solute is dissolved in a solvent, it will affect these properties the same way a different solute would, provided the number of solute particles is the same. Common colligative properties include:

• Vapor pressure lowering
• Boiling point elevation
• Freezing point depression
• Osmotic pressure

When we talk about freezing point depression, we're referring to the decrease in the freezing point of a liquid due to the presence of a solute. This phenomenon occurs because the solute particles disrupt the orderly freezing process of the solvent molecules, requiring more energy (lower temperature) to form a solid. Thus, solutions freeze at lower temperatures than pure solvents. Colligative properties are crucial in everyday applications — for instance, adding salt to ice on roads lowers the freezing point, preventing ice formation.

Phase Equilibrium

Understanding phase equilibrium is essential when discussing freezing point depression. Phase equilibrium occurs when the amount of a substance transitioning between two phases (e.g., liquid and solid) is constant over time, meaning the rate of freezing equals the rate of melting. In the context of freezing point depression, the equilibrium in question is between the liquid solvent and its solid form.
For instance, when a solution transitions to forming ice, the equilibrium is reached when the solution’s liquid phase continuously freezes at long-term equilibrium with the solid phase of the pure solvent. This equilibration continues until the temperature drops sufficiently enough to counterbalance the solute’s effect. When this equilibrium is disrupted by a solute, achieving this balanced state requires a lower temperature, which explains why the solution has a depressed freezing point compared to the pure solvent's freezing point.

Solvent and Solute Interactions

The interactions between solvent and solute particles are at the heart of freezing point depression. As a solute dissolves in a solvent, these interactions interfere with the regular lattice formation of solvent molecules necessary for them to solidify into a solid state. Let's break down why this happens:

• Disruption of Lattice Formation: Solute particles "get in the way," interrupting the organized structure that solvent molecules attempt to form as they freeze.
• Entropy Considerations: Adding solute increases the solution's overall disorder (entropy). Increased entropy makes it energetically less favorable for the solvent to form a solid.
• Salt on Ice Roads: A practical application of solvent-solute interaction, where added salt prevents water from forming solid ice by depressing the water's freezing point.

By understanding these interactions, students can grasp why a solute causes a lower freezing point in the mixture, highlighting the integral role of solvent-solute dynamics in colligative properties.