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

The correct answer is (c) 1,4, as depression in freezing point occurs and only solvent solidifies.

Step 1: Understanding Depression in Freezing Point

Depression in freezing point is a colligative property which occurs when a solute is added to a solvent, lowering the freezing point of the solvent. This is because the solute particles disrupt the lattice formation of the solid phase, thereby requiring a lower temperature to freeze. Thus, only solvent molecules solidify at this new, lower freezing point.

Step 2: Analyzing Vapour Pressure

When a solute is added to a solvent, the vapour pressure of the solution typically decreases. This is due to the presence of solute particles at the surface of the liquid, which reduces the number of solvent molecules that can escape into the vapor phase compared to the pure solvent.

Step 3: Identifying Correct Statements

From our understanding of the depression in freezing point and changes in vapour pressure: 1. The vapour pressure of the solution is less than that of the pure solvent. 4. At the freezing point, only solvent molecules solidify as solute particles interfere with the solidification process.

Key Concepts

Colligative Properties

Colligative properties are fascinating characteristics of solutions that depend only on the number of solute particles present in the solution, rather than their specific identity. Imagine if you dissolved anything from salt to sugar in water; the resulting changes in properties such as boiling point elevation, freezing point depression, and osmotic pressure would depend on how many particles you introduced. Colligative properties are influenced by the way solute particles disrupt or change the existing behavior of solvent molecules. For freezing point depression, when a solute is dissolved in a solvent, the freezing point of the solution becomes lower than that of the pure solvent. This occurs because solute particles block solvent molecules from arranging themselves into a solid structure as easily as they would in pure form. As a result, the temperature must be reduced further to allow the solvent to solidify, thus lowering the freezing point. This unique phenomenon is what makes everyday examples like adding salt to icy roads, so effective in lowering the freezing point to melt ice.

Vapour Pressure

Vapour pressure is a critical concept that describes the pressure exerted by a vapor when it is in equilibrium with its liquid or solid form. In simpler terms, it’s the tendency of particles to escape into the air. When a solute is added to a solvent, the vapour pressure of the solution usually decreases. This decrease occurs because solute particles take up space on the surface of the liquid. Less space means fewer solvent molecules can escape into the air, reducing the overall exerted pressure. Consider how this plays out in real-world scenarios: by adding a non-volatile solute, like sugar, into water, you effectively lower the number of water molecules that can reach the vapor phase. Consequently, the vapour pressure of the solution is always lower than that of the pure solvent. This principle explains why your pot of saltwater might take longer to boil than plain water.

Solute and Solvent Interaction

The interaction between solute and solvent molecules is foundational to understanding colligative properties and their effects on solutions. When you mix a solute with a solvent, the solute particles interact with the solvent molecules, altering how these substances behave. In the context of freezing point depression, solute molecules are more than just guests in the solvent’s home. They actively interrupt the orderly arrangement of solvent molecules necessary to form a solid or freeze. This disruption explains why solutes lower the freezing point, as more energy (in the form of a lower temperature) is required to overcome this interference and allow the solvent to solidify. This interaction isn’t merely theoretical; it has practical applications in everyday life, such as preventing freezing in car engines with antifreeze or creating delectable ice cream by freezing ingredients into a creamy solid at lower temperatures. The underlying principle is always the same: solute molecules alter the environment of the solvent, making certain physical transitions more challenging, like freezing.