Problem 134 - A Complete Resource Book in Chemistry for JEE Main

Question

Mercuric iodide is added to an aqueous solution of potassium iodide. Identify the correct statement(s) (a) Freezing point is raised. (b) Freezing point is lowered. (c) Freezing point and boiling point does not change. (d) Boiling point is raised. (a) 1 and 4 (b) 3 and 4 (c) Only 1 (d) 2 and 4

Step-by-Step Solution

Answer

(d) 2 and 4

Step 1: Understand the Initial Situation

We have an aqueous solution of potassium iodide (KI). When a solute is added to a solvent, colligative properties such as freezing point and boiling point are affected.

Step 2: Consider the Addition of Mercuric Iodide

Mercuric iodide (HgI₂) is added to the potassium iodide solution. This results in a new solute being present in the solution, increasing the overall number of solute particles in the solution.

Step 3: Evaluate the Effect on Freezing Point

Adding mercuric iodide to the solution will cause freezing point depression. This means that the freezing point of the solution will be lowered, as the presence of additional solute particles disrupts the formation of the solid phase.

Step 4: Evaluate the Effect on Boiling Point

Colligative properties also dictate that the boiling point of a solution is elevated when a solute is added. This means that the boiling point of the solution will be raised due to the addition of mercuric iodide.

Step 5: Compare to Given Statements

From the analysis, we see that statement (b) 'Freezing point is lowered' and statement (d) 'Boiling point is raised' accurately describe the effects observed when mercuric iodide is added to the potassium iodide solution.

Key Concepts

Freezing Point Depression

When a solute like mercuric iodide (HgI₂) is added to a solvent like an aqueous solution of potassium iodide (KI), the colligative property known as freezing point depression comes into play. Simply put, freezing point depression means that the temperature at which the solution freezes is lowered. The addition of solute particles interferes with the ability of the solvent molecules to organize themselves into a solid structure. As a result, the solution requires a colder environment, or lower temperature, to freeze.

• This is because the presence of solute particles disrupts the orderly arrangement of the liquid into a solid.
• The greater the number of solute particles in the solution, the more significant the depression of the freezing point.
• Freezing point depression is a colligative property, meaning it depends on the number of solute particles rather than their identity.

An excellent everyday example of this would be the use of salt to melt ice on roads during winter. By spreading salt, you're adding solute particles, effectively lowering the freezing point of water and preventing ice from forming easily.

Boiling Point Elevation

Boiling point elevation is another crucial colligative property observed when a solute is introduced to a solvent. In our context, adding mercuric iodide to the potassium iodide solution not only lowers the freezing point but also causes the boiling point to rise.

When a solute is dissolved in a solvent, it requires more energy for the solution to change from a liquid to a gas. Consequently, the boiling point of the solution increases. This occurs because solute particles attract and hold solvent molecules, preventing them from easily entering a gaseous state.

• The increase in boiling point is directly related to the number of solute particles present in the solution.
• Like freezing point depression, boiling point elevation is also a colligative property.
• It doesn’t matter what the solute is, just how many particles it produces in the solution.

This principle is applied in cooking where salt is added to water to increase its boiling point. The higher boiling point allows for cooking at slightly higher temperatures, potentially speeding up cooking times.

Solution Chemistry

Solution chemistry is the study of how different solutes interact with solvents to form solutions. It encompasses various fundamental concepts, including colligative properties like freezing point depression and boiling point elevation.

When mercuric iodide is added to the aqueous potassium iodide solution, it alters the solution's properties due to the increased number of solute particles.
In solution chemistry, it's essential to understand these interactions because they:

• Determine changes in physical properties such as melting and boiling points.
• Influence the solution's behavior, including aspects like vapor pressure.
• Are crucial in practical applications, from antifreeze in cars to preserving foods.

One key takeaway is that colligative properties depend primarily on how many solute particles are present, not the specifics of what those particles are. This universal characteristic makes solution chemistry incredibly insightful and applicable in both industrial and everyday contexts.