Comprehension:
Colligative Properties
Vapour pressure of a solution decreases when a non-volatile solute is added to a volatile solvent. There are many properties of solutions which are connected with this decrease of vapour pressure. These are: (1) relative lowering of vapour pressure of the solvent (2) depression of freezing point of the solvent (3) elevation of boiling point of the solvent and (4) osmotic pressure of the solution. 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.
An aqueous solution of sodium chloride has vapor pressure
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → less than that of water
By Raoult’s law, only the solvent contributes to the vapour pressure when the solute is non-volatile:
\( \displaystyle p_1=x_1\,p_1^\circ,\quad x_1<1\ \Rightarrow\ p_1<p_1^\circ \)
Sodium chloride is non-volatile, and it also dissociates, NaCl → \( \ce{Na^{+}} \) + \( \ce{Cl^{-}} \): 1 + 1 = 2 ions (i = 2), so the lowering is even larger than for the same amount of a non-electrolyte.
So an aqueous NaCl solution has a vapour pressure less than that of water.
Why the other options are incorrect
Option (1) equal to that of water — Incorrect. This would be true only if the solute had no effect; a dissolved non-volatile solute always lowers the vapour pressure.
Option (2) more than that of water — Incorrect. Only a volatile component can raise the vapour pressure; NaCl does not evaporate.
Option (4) equal to atmospheric pressure — Incorrect. The vapour pressure equals atmospheric pressure only at the boiling point; since it is lowered, the solution must be heated above 373 K to boil.
NCERT (2026-27 edition):
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. This vapour pressure of the solution at a given temperature is found to be lower than the vapour pressure of the pure solvent at the same temperature. …”
“The decrease in the vapour pressure of solvent depends on the quantity of non-volatile solute present in the solution, irrespective of its nature. …”