Is it a strict rule that when we boil a mixture of two liquids, with one being less volatile than the other, the more volatile vapour has to form the greater portion of the total vapours?

If it is so, how are we able to obtain a pure liquid of less volatility in the distillate and an azeotropic mixture by distilling a certain composition of a non-ideal mixture? Shouldn't we be getting ALWAYS the more volatile component in the higher plates and thus in the distillate?

And if it is not so, please enlighten me with a valid explanation. Thank you.

  • $\begingroup$ Greater volatility means that the fraction of vapour represented by the component will be greater than that expected on the basis of its mole fraction in the liquid, ie more volatile=mole fraction in the vapour > mole fraction in the liquid $\endgroup$ – Buck Thorn Sep 15 '19 at 18:32
  • $\begingroup$ The opening paragraph in this question suggests a need to for more effort, but the remainder of the question and the answer that it has triggered indicate that maybe this shouldn't have been put on hold. $\endgroup$ – Buck Thorn Sep 16 '19 at 5:57

The more volatile compound forms greater vapour portion, compared to its portion in the distilled liquid.

It forms e.g. 15% molar fraction of vapour, if there is 10% molar fraction in liquid.

In case of deviation from the Raoult law, when there is interaction between compounds, an azeotrope with minimal ( ethanol-water ) or maximal ( nitric acid water ) boiling point may be formed. This azeotrope acts like if it were the 3rd compound.

Within rectification columns , for each distillation stage upwards, the composition is slightly enriched with more volatile fraction.

And vice versa, for each distillation stage downwards, the composition is slightly enriched with less volatile fraction.

If we are interested just in more volatile fractions, the column bottom is attached to the vapour source input, what is typical for labs.

If we are interested in all fractions, what is typical in oil industry, the input is injected somewhere in the middle of column, where the equilibrium composition is the most similar to the input.

The more or less volatile fraction may be an azeotrop. Components in ratio forming the azeotropic ratio cannot be separated by destination, unless other components are added.

The typical example is the azeotrope ethanol water, denying to get more than about 95% ethanol by distillation, as this azeotrope has lower boiling point than pure ethanol.

Pure ethanol can be gained by adding benzene, forming the ternary azeotrope benzene - ethanol - water, with even lower boiling point.

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