Problem 133 - 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

Option (d), 2 and 4, are correct.

Step 1: Understanding the Effect on Freezing Point

When mercuric iodide \( \text{(HgI}_2) \) is added to an aqueous solution of potassium iodide \( \text{(KI)} \), it increases the concentration of solutes in the solution. Increasing the concentration of solutes lowers the freezing point of a solution due to the phenomenon called freezing point depression.

Step 2: Understanding the Effect on Boiling Point

Increasing the concentration of solutes in a solution elevates its boiling point. This is known as boiling point elevation. Thus, adding \( \text{HgI}_2 \) to \( \text{KI} \) solution will raise the boiling point of the solution.

Step 3: Evaluating the Options

Review the problem's options: - (a) states freezing point is raised (incorrect) - (b) states freezing point is lowered (correct) - (c) states no change in freezing or boiling point (incorrect) - (d) states boiling point is raised (correct) Thus, the correct answer should include statements that the freezing point is lowered and the boiling point is raised.

Key Concepts

Freezing Point Depression

When a solute is dissolved in a solvent, the resulting solution generally has a lower freezing point than the pure solvent. This phenomenon is known as freezing point depression. It occurs because the solute particles disrupt the formation of the orderly crystal lattice structure that is needed for the solvent to effectively freeze.
In simpler terms, adding a substance like mercuric iodide to water interferes with water molecules coming together, requiring even colder temperatures to freeze. This is why road salts are used in winter; they lower the freezing point of water, preventing ice from forming.

• The freezing point depression depends on the molality of the solution—that is, the number of solute particles per kilogram of solvent.
• It also depends on the van't Hoff factor, which accounts for the degree of ionization or dissociation of the solute in the solvent.
  

For example, if you were to add more mercuric iodide to the potassium iodide solution, you’d notice an even greater depression in the freezing point due to the increased concentration of solutes.

Boiling Point Elevation

Much like freezing point depression, boiling point elevation is another colligative property that arises when a non-volatile solute is dissolved into a solvent. In this case, adding a solute to a solvent increases the boiling point of the solution. Thus, you have to add more heat to make the solution boil as compared to the pure solvent. This happens because the solute particles essentially take up space at the surface, requiring more energy for the solvent molecules to escape into the vapor phase.
Boiling point elevation is determined similarly to freezing point depression and depends on the molality and the van't Hoff factor.

• For our exercise, adding mercuric iodide increases the boiling point of the potassium iodide solution.
• This means energy, in the form of heat, is required at higher temperatures for the solution to boil.
  

This can be thought of like adding sugar to water—once the sugar dissolves, the water requires more heat to reach its boiling point.

Solution Chemistry

Solution chemistry studies the interactions and properties of solutions and plays a crucial role in understanding colligative properties. Solutions are mixtures that result when a solute is dissolved in a solvent, forming a homogeneous mixture.
In the context of the exercise, the potassium iodide solution acts as the solvent, while mercuric iodide is the solute being added. When the solute is introduced, interactions between the solute and solvent molecules dictate the resulting properties such as freezing point depression and boiling point elevation.

• A critical aspect of solution chemistry involves observing how changes in temperature affect solubility and the properties of solutions.
• Dissolution and interaction mechanics can vary significantly based on chemical structure, temperature, and pressure.
  

Understanding these aspects helps predict how adding a particular solute to a solvent like water will modify its physical properties.