The liquids will give off molecules, until the atmosphere above the liquid has a vapour pressure equal to the respective temperature. For example, if the vapour pressure of a liquid was 760 mm Hg at 60° C, molecules would escape from the surface until there was a pressure of 760 mm Hg in the atmosphere vapour exerting pressure back at the liquid. When a solvent is enclosed, the liquid will evaporate until the partial pressure of the gas above the liquid equals the vapour pressure of the liquid. If some of the gaseous vapour is removed, more liquid will evaporate in order to equalize the vapour pressure and partial pressure. This is the principle behind distillation.
A liquid is heated in a distilling flask. The temperature of the liquid will increase (specific heat) until the vapour pressure/temperature of the first liquid is reached. At this point, all the heat energy (heat of vaporization) is used to evaporate the liquid. The hot vapour
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travels upward and reaches a condensing column which removes heat from the vapour. The gas recondenses back to a liquid and is collected in a receiving flask. Since this reduces the vapour pressure over the liquid in the distilling flask, more liquid is converted to vapour to equalize, which in turn recondenses. This is the general idea behind distillation. If there is only one liquid in the receiving flask, it will be separated from any nonvolatile solid. These solids would remain in the distillation flask. Thus, the liquid is purified.