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I'm aware that there is a similar question here but it didn't really answer my specific questions. I understand that steam distillation is used to separate an insoluble liquid from an aqueous solution, due to passing steam through the reaction mixture so they both evaporate etc...

I don't however understand what is so significant about this. If we started with a mixture of, let's say, 50% lavender oil and 50% water, wouldn't we also end up with a mixture of lavender oil and water, if both are evaporating below their boiling point? How is that useful - aren't we just ending up with what we started with? Or is it that the distillate will contain a greater proportion of oil? eg 60% to 40% - but then I don't see how that works, as shouldn't water should be more easily evaporated than the oil?

Furthermore, surely the two immiscible liquids form two seperate layers that can be put into a separating funnel and poured into separate containers?

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    $\begingroup$ Separating water from an organic compound is not the purpose of steam distillation. See the Wikipedia entry for "steam distillation" for a good background. $\endgroup$ – airhuff Jun 10 at 0:06
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    $\begingroup$ Possible duplicate of Steam distillation- how it actually works? $\endgroup$ – Mithoron Jun 10 at 22:49
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I think that the main purpose (/ historical advantage) of steam distillation was to avoid the exposure of sensitive compounds to sustained heat by either lowering the required distillation temperature, or the distillation duration (at equal temperature), or both, by using steam as a carrier.

The solution that contains the compound to be distillated doesn't necessarily initially contain water itself, though even if it does, there is a significant advantage in that the duration of exposure to high temperatures is as short as the transport time until it is condensed (since the water is boiled separately, and not together with the compound). In traditional distillation the compound has to be exposed to heat for the time necessary to reach the solvant's evaporation point AND the transport time, which is significantly longer than the transport time alone.

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