Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

When mercuric iodide is added to the aqueous solution of potassium iodide, the (a) freezing point is raised (b) freezing point does not change (c) freezing point is lowered (d) boiling point does not change

Step-by-Step Solution

Answer

The freezing point is lowered (option c).

Step 1: Understanding the Colligative Properties

Colligative properties are properties that depend on the number of particles in a solution. Freezing point depression is one such property, which refers to the lowering of the freezing point of a solvent due to the addition of a solute.

Step 2: Determining the Effect of Adding Mercuric Iodide

When mercuric iodide (HgI₂) is added to a solution of potassium iodide (KI), it dissolves in the solution, increasing the number of solute particles. This increase in solute particles will affect the colligative properties of the solution.

Step 3: Applying Freezing Point Depression Principle

According to the freezing point depression principle, adding a solute to a solvent lowers the freezing point of the solution compared to the pure solvent. This is because the presence of solute particles disrupts the formation of the solid crystal lattice of the solvent, requiring a lower temperature to freeze.

Step 4: Conclusion Based on Solute Addition

Given that the addition of mercuric iodide increases the solute particles in the aqueous solution of potassium iodide, the freezing point of the solution is lowered. Thus, the impact of adding mercuric iodide is a depression in the freezing point according to colligative property behavior.

Key Concepts

Colligative Properties

In chemistry, colligative properties are fascinating as they depend solely on the number of solute particles rather than the type of particles in a solution. This means that the effects observed depend on how many solute particles are present, not their identity. There are several well-known colligative properties:

• Freezing Point Depression
• Boiling Point Elevation
• Osmotic Pressure
• Vapor Pressure Lowering

Among these, freezing point depression is particularly interesting because it illustrates how adding solute particles can lower the freezing point of a solvent. When a solute like mercuric iodide is dissolved in a solution, it increases the total number of particles, which in turn impacts the colligative properties of the solution.

Solute-Solvent Interaction

Solute-solvent interaction is a key concept to understand when exploring colligative properties. When a solute is added to a solvent, it interacts at a molecular level. These interactions can interfere with the processes that allow the pure solvent to freeze or boil at its characteristic temperatures. The presence of solute particles disrupts the regular pattern of solvent molecules. Here, instead of the solvent forming a solid lattice—which is essential for freezing—there's a disruption caused by the scattered solute particles. Because of this disruption, the solution requires a lower temperature to reach freezing point compared to the pure solvent.

Effect of Solute on Freezing Point

A fascinating outcome of solute addition is the effect on the freezing point. The introduction of solute particles, such as when mercuric iodide is added to a potassium iodide solution, typically lowers the freezing point of the liquid.

Why Does This Happen?

The mechanism involves the solute particles getting in the way as the solvent molecules attempt to organize into a solid formation, which is essential for freezing. This interference means more kinetic energy must be removed for the solvent molecules to slow down and arrange properly to freeze. Hence, the freezing point drops. This principle is universal, explaining why roads are salted in winter. Salt disrupts water's crystal formation, keeping it liquid at temperatures where pure water would freeze. So, the impact of adding a solute is a lower freezing point, showcasing a fundamental colligative property.