Let's say I have a mixture of two miscible liquids. I want to get the liquids by fractional/pure distillation. My textbook says a mixture boils over a range of temperatures. On the contrary, another book I read says a mixture has only one boiling point and that the liquid with a lower boiling point adds more of itself to the mixture. I am wondering which of those is correct and will work with distillation.

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    $\begingroup$ The mixture boils over a range of temperatures. The lowest point of that range is called the boiling point. $\endgroup$ May 14, 2021 at 11:58
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    $\begingroup$ There is too few information to anticipate if a separation of the two liquids by distillation may be achieved, or not. Depending on their mutual interaction, perhaps they form an azeotrope and you only reach a maximal concentration of A in B (c /= 100%), e.g., HCl/water. $\endgroup$
    – Buttonwood
    May 14, 2021 at 12:12
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    $\begingroup$ The actual mixture (at a given x) has one defined boiling point (at given P) but as boiling change composition the boiling occurs over a range of T. $\endgroup$
    – Alchimista
    May 14, 2021 at 13:03
  • $\begingroup$ There is no unique answer to this question. A mixture water-ethanol may boil at a temperature between $78$°C and $100$°C, depending on the relative proportion of the constituants. A mixture of two non miscible liquids like oil + water boil at a given temperature, whatever the proportions of the constituants. $\endgroup$
    – Maurice
    May 14, 2021 at 20:37
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    $\begingroup$ It would be nice to know which books you are reading. $\endgroup$ May 15, 2021 at 9:03

1 Answer 1


I do use my comment above because the meaning of the books is, with little interpretation, clear and the answer is simple and don't require digging in theory of fractional distillation. So it would be pity to let the question unanswered.

A mixture as those discussed boils over a range of T in the sense that its boiling point BP depend on its composition.

The actual mixture, i. e. at a given molar ratio, has one well defined BP (at given P).

If you do not look at a given composition, but you actually conduct a distillation, as the boiling change composition the BP changes all along the process/column/plates, actually spanning a range of T.

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    $\begingroup$ so as the mixture boils, the composition changes and the boiling point also changes $\endgroup$ May 17, 2021 at 8:48
  • $\begingroup$ thanks I understand now $\endgroup$ May 17, 2021 at 8:49

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