Whereas ordinary distillation deals with the separation of miscible liquids, e.g. water, ethanol, methanol, etc., steam distillation deals with the separation of immiscible or partially miscible liquids, e.g. oil and water. When two immiscible liquids are heated, each exerts its own vapour pressure independently of the other. When the sum of the vapour pressures of the two liquids becomes equal to the atmospheric pressure, the two distil over together, and the temperature of distillation and the composition of the distilate remain constant until one of the liquids is entirely evaporated.
An example of how steam distillation works will be given, drawn from the literature, using water and chlorobenzene as the two liquids. A mixture of these two liquids was distilled when the atmospheric pressure was 740.2 mm of mercury. The mixture boiled at 90.3° C. At this temperature the vapour pressure of water is 530.1 mm Hg while that of chlorobenzene is 210.1 mm, making a total of 740.2 mm. Chlorobenzene has a boiling point of 132° C., yet when distilled with steam at a temperature 42° C. lower, the distilate contained over 70% of the organic compound.
Another example is aniline and water. Under the standard atmospheric pressure of 760 mm Hg a mixture of these two liquids boiled and distilled over at 98.5° C., at this temperature the vapour pressures of aniline and water being 43 mm and 717 mm respectively, for a total of 760 mm.
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