Problem 89 - A Complete Resource Book in Chemistry for JEE Main

Question

In a depression in freezing point experiment, it is found that (1) the vapour pressure of the solution is less than that of pure solvent (2) the vapour pressure of the solution is more than that of pure solvent (3) only solute molecules solidify at the freezing point (4) only solvent molecules solidify at the freezing point (a) 1,2 (b) 2,3 (c) 1,4 (d) 1,3

Step-by-Step Solution

Answer

Option (c) 1,4 is correct.

Step 1: Understanding the Freezing Point Depression

Freezing point depression is a colligative property that occurs when a solute is dissolved in a solvent, resulting in a lower freezing point compared to the pure solvent. This means that the presence of solute particles disrupts the formation of the solid phase (ice or crystal) of the solvent, thus lowering the freezing point.

Step 2: Vapour Pressure Relation with Solute and Solvent

When a non-volatile solute is added to a solvent, the vapour pressure of the solution decreases compared to that of the pure solvent because the solute particles occupy surface positions and reduce the number of solvent molecules that can escape into the vapour phase. Therefore, the statement where the vapour pressure of the solution is less than that of the pure solvent is correct (Option 1).

Step 3: Solidification Process at Freezing Point

At the freezing point of the solution, the solvent molecules primarily solidify because solute particles generally remain in the liquid phase. Solute molecules can only solidify if they reach saturation or specific conditions, which is not usually the case in a freezing point depression scenario. Thus, only solvent molecules solidify is the accurate statement (Option 4).

Step 4: Selecting the Correct Option

From the analysis, the accurate statements are: the vapour pressure of the solution is less than that of the pure solvent (1) and only solvent molecules solidify at the freezing point (4). Therefore, the correct option is (c) 1,4.

Key Concepts

Colligative Properties

Colligative properties are an interesting aspect of chemistry that depend solely on the number of solute particles in a solution, not their identity. Whether you're dealing with salt or sugar, if you add these to water, you'll notice changes that are purely due to the amount present, not what they are.
These properties include:

• Freezing point depression
• Boiling point elevation
• Vapour pressure lowering
• Osmotic pressure

When a solute is added to a solvent, the freezing point of the solution decreases compared to that of the pure solvent. This is called freezing point depression, a crucial aspect of colligative properties. The more solute particles added, the greater the freezing point drop.
Understanding these changes is vital in various applications, from antifreeze in cars to preserving food.

Vapour Pressure

Vapour pressure is defined as the pressure exerted by a vapour that is in equilibrium with its liquid or solid form. In simple terms, it's how many molecules are escaping from the liquid to the air.
When you put in a non-volatile solute, like sugar, it affects the vapour pressure by reducing it. This happens because the solute particles take up some of the space at the surface of the liquid. With fewer surface molecules that can escape into the air, the vapour pressure drops.

• This is why solutions often have a lower vapour pressure compared to their pure solvents.
• The effect is based on particle number, not type, demonstrating the essence of colligative properties.

Grasping this concept helps you understand why adding salt to water not only changes its taste but also its physical properties, proving invaluable in both day-to-day life and industrial processes.

Solidification Process

The solidification process in a solution during freezing involves only the solvent molecules, typically remaining clear of any solute. In the context of freezing point depression:

• The solvent molecules gather to form a solid, like ice, when cooled down.
• Solute particles stay dissolved in the liquid part because they disrupt the orderly formation of the solid phase.

For example, when salt is added to ice, it prevents the water molecules from arranging into a solid, which is why we use salt to melt ice on roads.
Understanding this process is crucial because it explains why solutions freeze at a lower temperature than pure solvents. Recognizing this principle has practical uses, such as preventing pipes from freezing in winter.

Solute and Solvent Interaction

Understanding the interaction between solute and solvent is essential in explaining many phenomena, including freezing point depression.
When a solute is introduced into a solvent, it disrupts the systematic arrangement of solvent molecules, altering their movement and interaction. This interaction changes several physical properties. Key points include:

• Solute particles interfere with the ability of solvent molecules to crystallize and form a solid.
• This disturbance in formation results in a lower freezing point for the solution.
• The vapour pressure is also affected because fewer solvent molecules are in the position to escape into the vapour phase.

In summary, when you dissolve something like sugar in water, you're not just sweetening your drink—you're changing its physical properties! These solute-solvent interactions help explain everyday occurrences, such as why salted roads are less icy in winter.