The vapour pressure of a two components system as a function of composition shows positive deviation from Raoult’s Law. Identify the correct curve to show the positive deviation
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → curve A
For an ideal solution, the partial vapour pressures p1, p2 and the total vapour pressure vary linearly with mole fraction (the straight dashed lines). In a solution with positive deviation, A–B interactions are weaker than A–A and B–B interactions, so molecules escape more easily and every vapour pressure is higher than the ideal value: the curves lie above the dashed lines (as for ethanol + acetone). In negative deviation they lie below.
Only in curve A do the partial and total vapour pressure curves all lie above the corresponding ideal straight lines.
NCERT (2026-27 edition):
Non-ideal Solutions – …
