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As given in the Wikipedia page,

An azeotrope (/əˈziːəˌtroʊp/) or a constant boiling point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.

It further goes on to give the reason;

This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture.

But why? Why do they have this special property? Why don't substances normally have the same composition in solid and liquid phase?

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  • $\begingroup$ It is a strange property, that nobody can really explain. Azeotropic mixtures behave like pure compounds. But they are not pure. The composition of azeotropic mixtures change with pressure. $\endgroup$ – Maurice May 8 at 15:10
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At the azeotropic point, the solution components are boiling off at the same rate proportional to their concentration. I.E. the composition of the vapor is the same as the composition of the solution.

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  • $\begingroup$ Functionally it's no different than pipetting out an aliquot of solution from the initial bulk stock - boiling just uses a phase change to achieve the same result. Does the concentration of a bulk solution change when you partition out an aliquot? Certainly not. So too does the concentration not change when you effectively do the same thing via vaporization. $\endgroup$ – Michael Green May 8 at 15:15
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    $\begingroup$ Your answer is correct. But it is not related to the question. The question is why ?Usually when heating mixtures of volatil liquids, the vapors do not have the same composition as the liquid. Azeotrops are exceptions. The question is : why ? $\endgroup$ – Maurice May 8 at 15:20

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