Minimum boiling azeotropes show ____ deviation from Raoult's Law at a specific composition.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Large Positive
An azeotrope is a binary mixture that has the same composition in the liquid and the vapour and boils at a constant temperature, so it cannot be separated by fractional distillation. Azeotropes are formed only by solutions with large deviations from Raoult’s law:
A large positive deviation means the A–B attractions are weaker than A–A and B–B. The vapour pressure is higher than expected, and at one composition it passes through a maximum. Highest vapour pressure means lowest boiling point, so this mixture is a minimum boiling azeotrope; for example, ethanol and water at about 95% ethanol by volume.
A large negative deviation gives a minimum in vapour pressure and so a maximum boiling azeotrope; for example, nitric acid and water at about 68% nitric acid by mass.
So minimum boiling azeotropes show a large positive deviation.
Why the other options are incorrect
Option (1) Small Positive — Incorrect. A small deviation does not produce a maximum in the vapour pressure curve, so no azeotrope forms.
Option (2) Small Negative — Incorrect. A small negative deviation gives neither a minimum vapour pressure nor an azeotrope.
Option (4) Large Negative — Incorrect. This gives the maximum boiling azeotrope, the opposite case.
NCERT (2026-27 edition):
Non-ideal Solutions – …
