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: Understanding Freezing Point Depression

Freezing point depression occurs when a solute is added to a solvent, decreasing the temperature at which the solvent transforms from liquid to solid. It's vital to understand that in this scenario, we focus on the state of the solvent when a non-volatile solute is dissolved.

Step 2: Identifying Equilibrium Conditions

In a solution experiencing freezing point depression, consider which states of matter are in equilibrium. Specifically, this involves both the solid and liquid phases of the solvent, since the solvent is what is freezing.

Step 3: Analyzing Solution Choices

Evaluate each choice: (a) Liquid solvent and solid solvent is the equilibrium within freezing point depression, as the liquid solvent begins to convert to its solid phase. (b) Liquid solvent and solid solute is not possible as solute primarily affects freezing point. (c) Liquid solute and solid solute doesn't relate because solutes are not creating the depression. (d) Liquid solute and solid solvent is incorrect as the solvent is not in the solute's liquid phase.

Step 4: Selecting the Correct Answer

Upon analyzing, the correct equilibrium state during freezing point depression is between the liquid solvent and its solid state. Thus, option (a), liquid solvent and solid solvent, is correct.

Key Concepts

Equilibrium in Solutions

Equilibrium is a fascinating concept in chemistry, where it implies a state of balance between opposing forces. In the context of solutions, this usually refers to the dynamic balance between the various phases of the substances involved. During freezing point depression, equilibrium is achieved when the rates of phase transition from solid to liquid and from liquid to solid are equal. This occurs specifically with the solvent, not the solute. As the solution cools, the solvent molecules organize into a solid structure while still interacting with the solute particles. This interaction effectively lowers the solvent's freezing point. If you are picturing how this looks in practice:

• The solid phase is growing steadily as the temperature decreases.
• Liquid molecules are less energetic, making it easier for them to join the solid phase.
• The system continues to adjust, maintaining equilibrium until a new phase transition point is reached.

Understanding this equilibrium helps explain why the addition of a solute leads to a lower freezing temperature.

Solvent and Solute Phases

In solutions, the solvent and solute can exist in different physical phases. The solvent is usually the liquid that dissolves the solute. During freezing point depression, it’s important to focus on the phase of the solvent, as this is the element affected by the added solute. Here's how it works:

• The solvent is in liquid form, surrounding solute particles.
• As temperature decreases, the solvent starts forming a solid phase.
• Despite forming a solid, some liquid solvent and solute remain, continuing to interact.

Imagine cooling a salty water solution: The liquid, now saturated with solute, starts to freeze at a lower temperature than pure water would. This happens because the solute molecules disrupt the solvent’s structure, forcing more energy to be lost for crystallization. The key to this process is understanding how the solvent is primarily responsible for the change in freezing point, with the solute altering this characteristic through disruption and interactions.

Phase Transition

Phase transition refers to the change of a substance from one state of matter to another, such as from liquid to solid. During freezing point depression, the phase transition is primarily observed in the solvent. This process is complex because the solution’s properties are altered by the solute. Let’s break it down:

• As temperature drops, solvent molecules start aligning into a solid lattice.
• The solute molecules hinder the organization of solvent molecules, delaying the solid formation.
• This interference is why the freezing point is "depressed" or lowered.

For example, consider how adding salt to icy roads lowers the freezing point of water, preventing freezing even at temperatures below zero degrees Celsius. This phenomenon is heavily utilized in real-world applications, showcasing the practical significance of phase transition knowledge in solutions. Knowing how solutes influence solvent phase changes can help in industries ranging from food preservation to road safety management.