Glucose is dissolved in water to make a solution. Which of the following contributes to the vapour pressure of this solution?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Water alone
A non-volatile solute such as glucose does not pass into the vapour. Only the solvent molecules (water) escape from the surface, so the vapour pressure of the solution comes from the solvent alone: \( p_1 = x_1 p_1^0 \).
Why the other options are incorrect
Option (1) Both glucose and water, in proportion to their mole fractions — Incorrect. This describes a solution of two volatile liquids, where each liquid gives its own partial pressure. Glucose has no vapour to contribute.
Option (2) Glucose alone — Incorrect. Glucose is the non-volatile part; it is the water that evaporates.
Option (4) Both glucose and water, in proportion to their masses — Incorrect. Glucose does not enter the vapour, and vapour pressure is related to mole fraction, not to mass.
NCERT (2026-27 edition):
1.4.3 Vapour Pressure of Solutions of Solids in Liquids – “…If a non-volatile solute is added to a solvent to give a solution, the vapour pressure of the solution is solely from the solvent alone.”
“…When the solute is non-volatile, only the solvent molecules are present in vapour phase and contribute to vapour pressure.”