Full Steam Ahead!

Boiling happens when the vapor pressure of the liquid (interior) equals the surrounding (exterior) pressure. In an open air process the exterior pressure is about one atmosphere. There are two ways to induce boiling: 1) increase the vapor pressure of the liquid by heating, 2) decrease the exterior pressure. In steam distillation the two immiscible liquids (water and oil) experience only part of the exterior pressure (Ptotal = pwater + poil = 1 atm) so the overall effect is like decreasing the exterior pressure for both liquids. Since water is present in relatively large amount and thus contributes the most to the total vapor pressure, its boiling point temperature will be reduced only a few degrees. The oily minor component, however, may experience a drastic shift in its boiling point temperature. Interestingly, the phenomenon has nothing to do with the oil being soluble in the steam. Also, you will notice that this is the opposite effect of the colligative property of boiling point elevation that occurs when water is mixed with a soluble/miscible substance.

Vapor pressures of two immiscible compounds and their mixture as functions of the temperature. http://www.uwlax.edu/faculty/koster/Distillation305.htm

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