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The below image (source: here) is from a fractional distillation of a mixture of two liquids, one with a boiling point of 69 degrees celsius and the other with a boiling point of 98 degrees celsius. In this diagram, it appears that the first drop of distillate occurs at the boiling point of the component with the lower component.

enter image description here

However, I was wondering why the first drop of distillate appears at the boiling point of the species with the lower boiling point, instead of being in between the boiling points of the two species.

My reasoning is here: If you start with a mixture of A and B, represented by point $a_1$, when you heat it, the first vapor will appear at point $a_2$ which corresponds to temperature $T_2$, which is in between the boiling points of either component.

Why does the first drop of distillate appear at a temperature equal to the boiling point of the lower boiling component, instead of in between the boiling points of the two components?

enter image description here

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  • $\begingroup$ Because the T is being read where the vapour condenses. $\endgroup$ – Alchimista Feb 6 at 8:23
  • $\begingroup$ Can you elaborate please? $\endgroup$ – Cyclopropane Feb 6 at 13:05
  • $\begingroup$ T in a distillation apparatus is that of the condensing phase. It is the T of the first drop of distillate. That is why fractional distillation works. $\endgroup$ – Alchimista Feb 7 at 7:47
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    $\begingroup$ Or in other words the T in the graph has nothing to do with the temperature of the liquid phase. Would the distillation have failed there will be no flat portion of the curve. Look for fractional distillation principle. Having no column would results in basically the situation you have described, in which you consider the T at which the liquid starts boiling. That is not the distilled phase. $\endgroup$ – Alchimista Feb 7 at 8:51

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