Chemistry · Solutions · NEET
An azeotrope is a mixture of two liquids that boils at a single, constant temperature and has the same composition in the liquid and in the vapour above it. Normally when you boil a mixture, the more volatile liquid escapes first, so the vapour is richer in one component. In an azeotrope this does NOT happen — the vapour has exactly the same ratio as the liquid. So the mixture behaves like a single pure substance while boiling.
Distillation works because the vapour is usually richer in the more volatile liquid than the liquid is. You collect that vapour, cool it, and slowly separate the liquids. But in an azeotrope the vapour and the liquid have the SAME composition. So no matter how many times you boil and condense, you always get back the same mixture. That is why NCERT says azeotropes cannot be separated by fractional distillation. For NEET, remember: same composition in both phases = no separation.
A minimum boiling azeotrope boils at a temperature LOWER than either pure liquid. It is formed by solutions that show large POSITIVE deviation from Raoult's law (weaker A-B forces, higher vapour pressure). Example: ethanol + water. A maximum boiling azeotrope boils at a temperature HIGHER than either pure liquid. It is formed by solutions that show large NEGATIVE deviation (stronger A-B forces, lower vapour pressure). Example: nitric acid + water. Memory link: higher vapour pressure means easier to boil = lower boiling point = minimum boiling.
Nitric acid + water is the standard NEET example of a maximum boiling azeotrope (about 68% HNO3 and 32% water by mass). It forms because HNO3 and water show large negative deviation from Raoult's law — the strong A-B attraction lowers vapour pressure, so the mixture boils at a temperature higher than either pure liquid. This exact idea was tested in NEET 2019.
Ethanol + water forms a MINIMUM boiling azeotrope. When you ferment sugars and distill, you can only reach about 95% ethanol by volume. At this azeotrope composition the liquid and vapour become identical, so further distillation cannot make it purer. Ethanol-water shows positive deviation from Raoult's law, and positive deviation → minimum boiling azeotrope.
Positive deviation from Raoult's law gives a MINIMUM boiling azeotrope. In positive deviation the A-B forces are weaker than A-A and B-B forces, so molecules escape more easily and vapour pressure rises above the ideal value. Higher vapour pressure means the mixture boils more easily, at a lower temperature — hence minimum boiling. Negative deviation (stronger A-B forces) does the opposite and gives a maximum boiling azeotrope.
The mixture that forms a maximum boiling azeotrope is:
The mixture which shows positive deviation from Raoult's law is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. An azeotrope boils at one constant temperature, just like a pure liquid. This is why it is sometimes called a 'constant boiling mixture'.
About 68% nitric acid and 32% water by mass, as given in NCERT. This is a maximum boiling azeotrope.
About 95% ethanol by volume. Fermentation and distillation cannot make ethanol purer than this by simple distillation because the mixture becomes an azeotrope.
Azeotropes arise from very large deviations from Raoult's law. Large positive deviation gives a minimum boiling azeotrope, and large negative deviation gives a maximum boiling azeotrope. Knowing the deviation lets you predict the azeotrope type in one step.
No. Ideal solutions obey Raoult's law over the whole range and show no deviation, so they cannot form azeotropes. Azeotropes only come from non-ideal solutions with large deviations.
