What type of azeotropic mixture and the deviation from Raoult's law does the binary mixture of ethanol and water (at specific composition) show?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Minimum boiling azeotrope, positive deviation
The type of an azeotrope depends on the deviation from Raoult’s law:
Positive deviation: A–B attractions are weaker than A–A and B–B attractions. Molecules escape more easily, the vapour pressure is higher than expected, so the boiling point is lower: a minimum boiling azeotrope.
Negative deviation: A–B attractions are stronger. The vapour pressure is lower than expected, so the boiling point is higher: a maximum boiling azeotrope.
In pure ethanol, molecules are held by hydrogen bonds. When water is added, it breaks many of these hydrogen bonds, so the mixture has a higher vapour pressure than predicted, a positive deviation. The mixture of about 95% ethanol by volume forms a minimum boiling azeotrope (boiling at about 351 K).
Why the other options are incorrect
Option (2) Maximum boiling azeotrope, negative deviation — Incorrect. This describes mixtures like nitric acid and water (about 68% \( \ce{HNO3} \), boiling at 393.5 K), not ethanol and water.
Option (3) Maximum boiling azeotrope, positive deviation — Incorrect. A positive deviation raises the vapour pressure, which always lowers the boiling point, so this pair is impossible.
Option (4) Minimum boiling azeotrope, negative deviation — Incorrect. A negative deviation lowers the vapour pressure, which raises the boiling point, so this pair is impossible.
NCERT (2026-27 edition):
Non-ideal Solutions – “… Some liquids on mixing, form azeotropes which are binary mixtures having the same composition in liquid and vapour phase and boil at a constant temperature. In such cases, it is not possible to separate the components by fractional distillation. There are two types of azeotropes called minimum boiling azeotrope and maximum boiling azeotrope. The solutions which show a large positive deviation from Raoult’s law form minimum boiling azeotrope at a specific composition. For example, ethanol-water mixture (obtained by fermentation of sugars) on fractional distillation gives a solution containing approximately 95% by volume of ethanol. Once this composition, known as azeotrope composition, has been achieved, the liquid and vapour have the same composition, and no further separation occurs.”