Organic substances are rarely made pure in one single process. Usually the first preparation results in a mixture containing a host of unwanted substances which have to be removed at a later stage.
Distillation often provides the answer. But ordinary distillation has certain drawbacks. Crude chlorobenzene, (C6H5Cl), with its many tarry impurities, presents this problem. Its boiling point is high (132°C), and therefore it must be heated strongly for distillation to begin. Some of the impurities start to char when it is strongly heated. Instead of boiling and bubbling gently, the contents start erupting and bumping and the far-too-hot flask could be cracked by a particularly violent bump.
Instead of heating the distillation flask with a bunsen burner, the flask can be successfully heated by bubbling steam through its contents. The steam keeps the mixture well stirred and at a temperature of around 90°C, a watery mixture of chlorobenzene distills off, leaving the impurities behind. The chlorobenzene is insoluble in water and the two quickly separate out into two easily separated layers.
The crux of the matter is the insolubility of the chlorobenzene in water. Steam distillation will only work for substances that are virtually insoluble in water. For when two substances are immiscible in one another they are capable of quite independent behavior.
Water and chlorobenzene are immiscible. Water on its own, when it is heated, evaporates more and more until at 100°C the pressure of its vapor has risen to that of its surroundings and the water boils. Chlorobenzene on its own behaves similarly, but boils at the higher temperature of 132°C. At a particular temperature, both pure liquids exert their own particular vapor pressures.
Mixing chlorobenzees with water will not alter these boiling points, for the two substances behave independently. At a particular temperature, the vapor pressure of the mixture can be calculated by adding the vapor pressure of the water to that of the chlorobenzene. So the mixture being heated more quickly arrives at the pressure of its surroundings than would either pure chlorobenzene or pure water. And the boiling point of the mixture is therefore lower than that of pure chlorobenzene or pure water. Steamy chlorobenzene is driven over into the condenser.
Related category PHYSICAL CHEMISTRY
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