Question

Abnormal colligative properties are observed only when the dissolved non-volatile solute in a given dilute solution

• A. is a non-electrolyte
• B. offers an intense colour
• C. associates and dissociates
• D. offers no colour

Hint:

When studying colligative properties, remember that the key factor is the number of particles in solution. Association or dissociation of solute particles can lead to deviations from expected behaviour.

Answer 1

The Correct Option is C

Abnormal colligative properties occur when the behavior of a solution deviates from what is predicted by the ideal solution model. Colligative properties depend only on the number of particles in a solution, not their identity. These properties include boiling point elevation, freezing point depression, osmotic pressure, and vapor pressure lowering.

For a given dilute solution:

• If the solute is a non-electrolyte, it does not dissociate into ions; thus, the colligative properties are normal and as expected.
• If the solute offers an intense colour or offers no colour, this characteristic does not directly influence colligative properties.
• If the solute associates and dissociates, it can either increase or decrease the number of particles, leading to abnormal colligative properties. For example, if a solute dissociates into multiple ions, the number of solute particles increases, affecting properties like boiling point or freezing point more than expected (e.g., NaCl dissociating to Na⁺ and Cl^-). Conversely, if association occurs (where molecules come together), the number of effective particles decreases, resulting in underestimation of such properties.

Thus, the correct answer is the option where the solute associates and dissociates, as it is this behavior that leads to abnormal colligative properties.

Answer 2

Abnormal colligative properties occur when the behavior of the solution deviates from the expectations based on the concentration of solute particles alone. Colligative properties depend on the number of solute particles in a solution, not their identity. These properties include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

For a non-volatile solute, abnormal colligative properties arise due to either:

1. Association: When molecules aggregate, reducing the effective number of solute particles. This is common in solutions of certain organic acids and results in lower than expected colligative properties.
2. Dissociation: When solute molecules separate into ions, increasing the number of particles and thus enhancing the colligative properties. This occurs in ionic compounds like salts in solution.

Therefore, in the context of this question, unusual colligative properties are observed when the dissolved non-volatile solute associates and dissociates.