Azeotropic Mixtures

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In case of solutions showing positive deviation from Raoult's law, at one of the intermediate compositions, the total vapor pressure is the highest and the boiling point is the lowest. Similarly in case of solutions showing negative deviations from Raoult's law, at one of the intermediate compositions, the vapor pressure is the lowest and boiling point is the highest. For such solutions the composition in the liquid and the vapor phase is the same. In other words such solutions vaporizes without any change in the composition i.e., they behave as pure liquid.

Such mixture of liquids that boil at constant temperature behaving like a pure liquid, and its composition in liquid phase is same as in vapor phase, is called Azeotropic mixture.

Solutions that show positive deviation form minimum boiling azeotropy.

Solutions of ethanol and water show a large negative deviation from Raoult's law and there exists a minimum boiling mixture which consists of 95% of ethanol by volume. To separate the components of this mixture distillation cannot be used.

fig 3.8 - The boiling point diagram for ethanol-water (not-drawn to scale)

Solution that show negative deviation form maximum boiling azeotropy.

A solution of HNO₃ and H₂O of approximately 68% nitric acid and 32% of water by mass is an example of maximum boiling azoetrope.