Addition of sodium chloride to water to form a solution would  lead to?

Analysis of the Question

This question tests your understanding of Colligative Properties of solutions. Specifically, it asks how the addition of a non-volatile solute (sodium chloride, NaCl) affects the physical properties of a solvent (water).

Concept Explanation

Colligative properties are properties of a solution that depend on the number of solute particles present, rather than the nature of the solute itself. When you add a solute like salt to a pure solvent like water, two major changes occur:

1. Boiling Point Elevation: The presence of solute particles interferes with the solvent molecules escaping into the vapor phase. To reach the vapor pressure required for boiling, the solution must be heated to a higher temperature than the pure solvent. Therefore, the boiling point increases.
2. Freezing Point Depression: The solute particles disrupt the formation of the ordered crystalline structure of the solid solvent. Because it is harder for the solvent to freeze, the temperature must be lowered further to turn the solution into a solid. Therefore, the freezing point decreases.

Step-by-Step Solution

1. Identify the solute and solvent: Sodium chloride (NaCl) is the solute, and water ($H_2O$) is the solvent.
2. Apply Boiling Point Elevation: Adding NaCl to water makes it harder for water to boil. Thus, the boiling point of the solution will be higher than $100^\circ C$. This is an increase.
3. Apply Freezing Point Depression: Adding NaCl to water makes it harder for water to freeze. Thus, the freezing point of the solution will be lower than $0^\circ C$. This is a decrease.
4. Combine the findings: We are looking for an option that states a decrease in freezing point and an increase in boiling point.

Comparing this to your options:

• Option A: Incorrect (Freezing point doesn't increase).
• Option B: Incorrect (Both are wrong).
• Option C: Incorrect (Boiling point doesn't decrease).
• Option D: Correct (Matches our findings).

Exam Tip

Think of the "Salt on the Road" trick: In cold countries, salt is spread on icy roads during winter. Why? Because the salt lowers the freezing point of the ice, causing it to melt at temperatures where it would normally remain frozen. This helps you remember that adding salt decreases the freezing point!