The figure shows how the vapour pressure of a pure solvent and of a solution of a non-volatile solute in it change with temperature.
A second solution of the same solute in the same solvent is more concentrated than the solution shown. Where would its vapour pressure curve lie, and at what temperature would it boil at 1.013 bar?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Below the curve of the solution shown; it would boil above Tb.
The decrease in the vapour pressure of the solvent depends on the quantity of non-volatile solute present. A liquid boils when its vapour pressure becomes equal to the atmospheric pressure, 1.013 bar.
Position of the curve — below the solution curve. More solute lowers the vapour pressure more, so at every temperature the more concentrated solution has a lower vapour pressure than the solution shown.
Boiling point — above Tb. In the figure the solution curve reaches the 1.013 bar line at Tb. A curve lying lower reaches that line only at a higher temperature, so ΔTb is larger.
Why the other options are incorrect
Option (1) Above the curve of the solution shown; it would boil between Tb0 and Tb. — Incorrect. A curve above the one shown would mean a smaller lowering of vapour pressure, which fits a more dilute solution, not a more concentrated one.
Option (3) On the same curve as the solution shown; it would boil at Tb. — Incorrect. The lowering of vapour pressure depends on the quantity of solute, so solutions of different concentration have different curves.
Option (4) Above the curve of the pure solvent; it would boil below Tb0. — Incorrect. A non-volatile solute always lowers the vapour pressure, so no such solution lies above the solvent curve or boils below the boiling point of the pure solvent.
NCERT (2026-27 edition):
1.4.3 Vapour Pressure of Solutions of Solids in Liquids – “The decrease in the vapour pressure of solvent depends on the quantity of non-volatile solute present in the solution, irrespective of its nature.”
1.6.2 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, 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).”
Fig. 1.7: The vapour pressure curve for solution lies below the curve for pure water. The diagram shows that ΔTb denotes the elevation of boiling point of a solvent in solution.