Why is the increase in the viscosity of a solution on adding a solute not considered to be a colligative property?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The resultant viscosity depends on the nature of the solute
A colligative property depends only on the number of solute particles in the solution, not on what those particles are. One mole of sucrose or one mole of urea in the same mass of water lowers the freezing point by the same amount.
Viscosity does not behave like this. Two solutes present in the same number of moles can raise the viscosity of water by very different amounts, because the effect depends on the size, shape and attractions of the solute molecules. Since the rise in viscosity depends on the nature of the solute, it is not a colligative property.
Why the other options are incorrect
Option (2) The resultant viscosity depends on the amount of solute — Incorrect. Colligative properties also depend on the amount of solute (more particles give a larger effect), so this cannot be the reason viscosity is excluded.
Option (3) The resultant viscosity depends on the nature of the solvent — Incorrect. Colligative properties also depend on the solvent: the constants Kf and Kb differ from one solvent to another. Only the nature of the solute must not matter.
Option (4) The resultant viscosity depends on the amount of solvent — Incorrect. Colligative properties depend on the concentration, that is, the solute particles relative to the amount of solvent, so this is not what separates viscosity from them.
NCERT (2026-27 edition):
1.6 Colligative Properties and Determination of Molar Mass – “… All these properties depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution. Such properties are called colligative properties (colligative: from Latin: co means together, ligare means to bind).”