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

(c) freezing point is lowered

Step 1: Understand the Context

When a solute is added to a solvent, it can affect the solution's colligative properties, namely freezing point and boiling point.

Step 2: Identify the Solute and Solvent

In this exercise, mercuric iodide is added to aqueous potassium iodide. Hence, mercuric iodide is the solute, and the aqueous potassium iodide solution is the solvent.

Step 3: Determine the Effect on Freezing Point

Adding a solute such as mercuric iodide to an aqueous solution of potassium iodide will lower the freezing point of the solution, due to the phenomenon known as freezing point depression.

Step 4: Conclusion

Given that adding mercuric iodide lowers the freezing point, the correct option is (c) freezing point is lowered.

Key Concepts

Freezing Point Depression

When you add a solute to a solvent, one of the noticeable changes is in the freezing point of the solution. This phenomenon is known as freezing point depression. It is a colligative property, which means it is dependent on the number of solute particles in the mixture, not the type. When mercuric iodide (the solute) is added to the aqueous solution of potassium iodide (the solvent), the freezing point of this solution decreases. This is because the solute particles disturb the natural formation of a solid structure in the solvent.
The more solute particles you have, the lower the temperature at which the solution will freeze. Freezing point depression occurs in various kinds of solutions, and its practical applications include de-icing roads using salt and the antifreeze used in car radiators.

Solvent-Solute Interaction

The interaction between the solute and the solvent is the key to understanding how and why the properties of the solution change. Different solute particles can dissolve in a solvent, creating a new phase of matter. When a solute like mercuric iodide is introduced into an aqueous potassium iodide solution, it affects how the solvent molecules interact with each other.
Here's why this matters:

• The presence of solute particles makes it harder for solvent molecules to line up and form a solid structure, which is why the freezing point drops.
• The disrupted interactions require more energy (lower temperature) to re-establish the organized structure of a solid.

This interaction is vital for many applications in chemistry and industry, as it helps us manipulate solutions according to our needs.

Aqueous Solutions

An aqueous solution is simply a solution where water acts as the solvent. Water is known for being a "universal solvent" due to its ability to dissolve many substances. The ability of water to dissolve various chemicals is critical in biochemical processes and industrial applications.
Aqueous solutions have specific properties:

• They play a crucial role in many chemical reactions and biological processes.
• Properties like boiling point, freezing point, and vapor pressure can change with the presence of a solute.
• Aqueous solutions are used in countless applications, such as in medicine, biology, and chemical manufacturing.

Understanding aqueous solutions provides insight into numerous scientific and practical applications, from how our bodies function to the design of commercial products.