Adding a salt to water leads to increase in the boiling point of solution with respect to water because:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Vapour pressure of solution is decreased
A liquid boils when its vapour pressure becomes equal to the external (atmospheric) pressure. When a non-volatile solute such as a salt is dissolved in water, part of the surface is occupied by solute particles, fewer water molecules escape, and the vapour pressure of the solution is lower than that of pure water (Raoult’s law). The solution must therefore be heated to a higher temperature before its vapour pressure reaches the external pressure, so its boiling point rises:
\( \displaystyle \Delta T_b = K_b\,m \)
Why the other options are incorrect
Option (1) Vapour pressure of solution is increased — Incorrect. A higher vapour pressure would make the solution boil at a lower temperature.
Option (2) Solubility of salt in water is increased — Incorrect. Solubility does not decide the boiling point; the amount of solute dissolved (molality) does.
Option (3) Solubility of salt in water is decreased — Incorrect. A change in solubility only alters how much salt can dissolve; the rise in boiling point comes from the lowered vapour pressure of the solution.
NCERT (2026-27 edition):
Elevation of Boiling Point – “The vapour pressure of a liquid increases with increase of temperature. It boils at the temperature at which its vapour pressure is equal to the atmospheric pressure. For example, water boils at 373.15 K (100° C) because at this temperature the vapour pressure of water is 1.013 bar (1 atmosphere). We have also learnt in the last section that vapour pressure of the solvent decreases in the presence of non-volatile solute. Fig. 1.7 depicts the variation of vapour pressure of the pure solvent and solution as a function of temperature. For example, the vapour pressure of an aqueous solution of sucrose is less than 1.013 bar at 373.15 K. In order to make this solution boil, its vapour pressure must be increased to 1.013 bar by raising the temperature above the boiling temperature of the pure solvent (water). Thus, the boiling point of a solution is always higher than that of the boiling point of the pure solvent in which the solution is prepared as shown in Fig. 1.7.”
