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.

Step 1: Understanding the Depression in Freezing Point

In a depression of the freezing point experiment, a solute is added to a solvent, resulting in the freezing point of the solution being lower than that of the pure solvent. This phenomenon occurs because the solute disrupts the orderly arrangement of the solvent molecules needed for crystallization, requiring a lower temperature to freeze.

Step 2: Analyzing Vapour Pressure Conditions

Adding a non-volatile solute to the solvent decreases the solution's vapor pressure compared to the pure solvent. Hence, the statement that the vapor pressure of the solution is less than that of the pure solvent (Option 1) is correct, whereas the statement that the vapor pressure of the solution is more than that of the pure solvent (Option 2) is incorrect.

Step 3: Examining Freezing Point Solidification

At the freezing point of a solution, it is primarily the solvent molecules that solidify, not the solute. This is because the solute particles interfere with the structure of the solvent crystals. Therefore, the statement that only solute molecules solidify (Option 3) is incorrect, while the correct statement is that only solvent molecules solidify (Option 4).

Step 4: Identifying the Correct Answer

Based on the analysis, option 1 is correct and option 4 is also correct. Hence, the correct combination from the given choices is option (c) 1,4.

Key Concepts

Vapor Pressure

Vapor pressure is an important concept in understanding how solutions behave compared to their pure solvents. When a non-volatile solute is added to a solvent, it causes a decrease in the vapor pressure of the solution. This occurs because the solute molecules occupy space at the surface, reducing the number of solvent molecules that can escape into the vapor phase.

As a result, the vapor pressure of the solution becomes less than that of the pure solvent. Lower vapor pressure means that the liquid needs less tendency to evaporate. This effect is a fundamental principle of colligative properties, which are properties of solutions that depend on the number of solute particles.

• Non-volatile solutes reduce vapor pressure.
• Vapor pressure is lower than that of a pure solvent.
• Colligative properties are influenced by solute quantity, not type.

The lowering of vapor pressure is directly related to why freezing point depression occurs, as the decreased tendency for solvent molecules to enter the gas phase affects their ability to solidify.

Solute and Solvent Interactions

Solute and solvent interactions significantly impact the physical properties of solutions, such as freezing and boiling points. When a solute is dissolved in a solvent, it prevents the solvent molecules from arranging into a solid structure necessary for freezing. This interference with the orderly crystallization process is why solutions typically freeze at lower temperatures compared to pure solvents.

The interactions between solute and solvent molecules create what is called "freezing point depression." This means that more energy (in the form of lower temperatures) is required to overcome the disruption and force the solvent into a solid state. Some key points about solute-solvent interactions are:

• Solute disrupts solvent's crystalline structure.
• Greater solute concentration leads to greater depression.
• Only solvent molecules solidify at the solution's freezing point.

Understanding these interactions helps explain why solutions behave differently than their pure components and are essential in predicting and modifying solution behaviors in various scientific and industrial applications.

Solidification at Freezing Point

At the freezing point, a solution undergoes a phase change from liquid to solid; however, this process is influenced by the presence of a solute. Primarily, it is only the solvent molecules that form a solid structure. This is because the solute molecules, being dispersed among the solvent, disrupt the lattice formation necessary for freezing.

This is why solutions freeze at lower temperatures and is one of the key aspects observed in a typical freezing point depression experiment. Some important aspects of solidification at freezing point are:

• Only the solvent solidifies because its molecules must gather in a precise order.
• Solute interferes with this orderly structuring, lowering the freezing point.
• Application in real-world: salt is used on icy roads to lower the freezing point, preventing ice formation.

Solidification at the freezing point highlights the impact of solutes on phase transitions. By manipulating solute interactions, industries can control freezing processes, enhancing product performance and safety across various contexts.