Problem 177 - A Complete Resource Book in Chemistry for JEE Main

Question

Assertion: Colloidal solutions show colligative properties Reason: Colloidal particles are large in size

Step-by-Step Solution

Answer

The assertion is true, but the reason is false.

Step 1: Understanding Colligative Properties

Colligative properties are properties that depend on the number of solute particles in a solution, not on the nature of the solute. Examples include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

Step 2: Characteristics of Colloidal Solutions

Colloidal solutions consist of particles that are dispersed in a medium where the particle sizes range from 1 to 1000 nanometers. These particles are larger than typical solute molecules in true solutions but are small enough to remain suspended indefinitely.

Step 3: Assertion Analysis

The assertion states that colloidal solutions show colligative properties. Since colligative properties depend on the number of particles and colloidal solutions have dispersed particles, they should exhibit some colligative properties, though the effect is less pronounced than in true solutions due to lower particle numbers.

Step 4: Reason Evaluation

The reason given is that colloidal particles are large in size. While it is true that colloidal particles are larger than molecules in typical solutions, the size is not the determining factor for colligative properties; it is the number of particles that matters.

Step 5: Conclusion from Analysis

The assertion is correct that colloidal solutions display colligative properties, albeit to a lesser extent. However, the reason provided is not valid as colligative properties are based on the quantity of solute particles.

Key Concepts

Colloidal Solutions

A colloidal solution, often simply called a colloid, is a type of homogeneous mixture where one substance is dispersed evenly throughout another. What makes these mixtures unique is the size of the particles involved. In colloids, particles range in size from 1 to 1000 nanometers, which is larger than individual ions or molecules typically found in true solutions but still small enough to remain evenly distributed without settling out.
In a colloidal solution, the dispersed particles are large enough to scatter light, a phenomenon known as the Tyndall effect. This is why when you shine a beam of light through a colloidal solution, you can often see the path of the light beam. Common examples of colloids include milk, mayonnaise, and fog. Despite their large particle size compared to true solutions, these dispersed particles can sometimes exhibit colligative properties, though the effects are often less noticeable due to fewer numbers of particles compared to those of molecular size in true solutions.

Particle Size

The particle size in colloidal solutions plays a crucial role in determining their properties and behavior. Unlike solutions with smaller-sized solute particles, colloidal particles stay suspended because their sizes are in the range of 1 to 1000 nanometers. This size range ensures that colloidal particles are small enough to be affected by the thermal movement of the molecules in the dispersion medium yet large enough to not dissolve completely.
This distinct characteristic of colloidal particles leads to their ability to exhibit the Tyndall effect, giving colloids a unique optical property. The size of colloidal particles often affects how they interact with light or other substances. Despite their relatively large size, the number of particles in colloidal solutions is lower than true solutions, which influences their ability to show colligative properties.

Boiling Point Elevation

Boiling point elevation is one of the colligative properties that describes how the boiling point of a solvent increases when a solute is dissolved in it. This happens because the presence of solute particles disrupts the solute structure, requiring more energy (heat) to reach the boiling point.
However, in colloidal solutions, the effect of boiling point elevation is much weaker compared to true solutions. This is because the number of dispersed colloidal particles is often fewer than the solute particles in a true solution. Therefore, while colloidal solutions can show this property, it is not as significant or easily measurable.

Freezing Point Depression

Freezing point depression is another colligative property where the freezing point of a solvent is lowered by the addition of a solute. The solute particles interfere with the formation of the solid lattice structure of the solvent, thus requiring a lower temperature to reach the solid phase.
In colloidal solutions, freezing point depression can occur but is generally less noticeable than in true solutions. This is due to a smaller number of colloidal particles affecting the freezing process. For colloids, even though their particle size is larger, it is the number of particles that primarily determines the extent of freezing point depression, much like all colligative properties. Whether in boiling point elevation or freezing point depression, the main takeaway is that the changes in phase temperatures depend more on the concentration of particles rather than their size.