Why is the ‘raising of viscosity’ of a solution after the addition of solute, not considered to be a colligative property?

The resultant viscosity depends on the nature of the solute

The correct answer is option 1. The resultant viscosity depends on the nature of the solute.

Let us delve into why viscosity is not considered a colligative property and why the correct answer is: The resultant viscosity depends on the nature of the solute.

Colligative properties of solutions are those that depend solely on the number of solute particles present in the solution, regardless of their chemical identity. These properties include:

Vapor pressure lowering: Adding a non-volatile solute decreases the vapor pressure of the solvent.

Boiling point elevation: The presence of solute particles raises the boiling point of the solvent.

Freezing point depression: Adding solute particles lowers the freezing point of the solvent.

Osmotic pressure: This is the pressure required to stop osmosis across a semipermeable membrane, which is dependent on the concentration of solute particles.

These properties are called colligative because they are affected only by the number of solute particles, not by their specific chemical nature. For instance, whether you add glucose or sodium chloride to water, the degree of freezing point depression or osmotic pressure increase will be directly proportional to the number of particles (moles) of solute added, assuming ideal behavior.

On the other hand, viscosity is a measure of a fluid's resistance to flow. When a solute is added to a solvent, it can interact with the solvent molecules in various ways depending on its chemical structure. These interactions can affect how the solvent molecules move past each other, thus altering the viscosity of the solution. For example:

Nature of the solute: Different solutes can form different types of interactions with the solvent molecules (like hydrogen bonding, dipole-dipole interactions, etc.), which can increase or decrease the viscosity of the solution.

Amount of solute: While colligative properties are influenced by the number (concentration) of solute particles, viscosity is also influenced by the total amount of solute added, affecting the overall interaction between solvent and solute molecules.

Therefore, viscosity is not solely dependent on the number of solute particles (as colligative properties are), but also on the specific interactions between the solvent and solute molecules. This dependency on the nature (chemical identity) of the solute makes viscosity a non-colligative property of solutions. Hence, the correct answer is that the resultant viscosity of a solution depends on the nature of the solute.