The mole fractions of the components of vapours aren't the same as those of the liquid-liquid solution (unless the solution is azeotropic), this implies a change in the composition of the liquid solution when vapours arise. Consequently, the partial vapour pressures of the components would also change and there would be a messed up cycle. There has to be something that I'm understanding wrongly and I'd like to be told what it is.

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    $\begingroup$ Yes, the composition of the liquid does change. However, if you carry out it in a closed vessel the compositions would reach an equilibrium $\endgroup$ – Shoubhik R Maiti Apr 25 at 19:57
  • $\begingroup$ But a liquid cannot boil in a sealed container..? $\endgroup$ – Bhavya Minocha Apr 25 at 20:00
  • $\begingroup$ Well you can heat it in a closed container. If you mean "boil" in the sense of boiling off vapour, then yes it would have to be an open container, and definitely the composition of the liquid will change over time. $\endgroup$ – Shoubhik R Maiti Apr 25 at 20:01
  • $\begingroup$ Distillation, right? $\endgroup$ – Ed V Apr 25 at 20:06
  • $\begingroup$ Here is a textbook chapter: chem.libretexts.org/Bookshelves/… $\endgroup$ – Karsten Theis Apr 26 at 0:56

It is not a messed up cycle. Let's consider what happens with an example.

If you heat up a mixture containing $10$% ethanol + $90$% water, the mixture boils at $92$°C, producing a vapor containing $50$% ethanol + $50$% water. With this operation the liquid loses more ethanol than water. So its concentration in ethanol decreases, and it is necessary to heat it up to a higher temperature to maintain the process of evaporation. When about one tenth of the liquid has distilled, the residual liquid contains still $5$% ethanol, and it is necessary to heat it up to $95$°C to obtain and continue boiling. At $95$°C, the vapor emitted contains $35$% ethanol. At $98°$C, the liquid contains still $2$% ethanol and the vapor $20$% ethanol. When the last drops are vaporized, the temperature is $100$°C and the last drops are made of pure water.

  • $\begingroup$ (+1) for the distillation example! $\endgroup$ – Ed V Apr 25 at 20:40
  • $\begingroup$ I was actually studying fractional distillation when the problem arose. When distilling ethanol, the solution of a given concentration is boiled and it's vapours extracted (and condensed). What confused me was this- how do we get a particular composition of vapours while boiling a solution (which is essential to obtaining a higher concentration of ethanol)? Or to do so, are the vapours extracted from a closed vessel rather than from a constantly boiling solution? Your answer gives an outlook of moving 'up the curve' in a positive azeotrope phase diagram, which I now fully understand. $\endgroup$ – Bhavya Minocha Apr 26 at 5:26

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