Simple distillation does not give a pure liquid if the boiling points of the liquids in the mixture are very close to each other (difference less than 40 K). In such a case, simple distillation gives a distillate that is not pure but contains both the liquids. Using fractional distillation we can purify such mixtures.
Fractional distillation makes use of a fractionating column of a suitable length and shape. A fractionating column is a long tube having different shapes and designs to suit particular requirements. All fractionating columns provide a large surface area where vapours can condense and come in contact with the incoming vapours.
For fractional distillation, a suitable fractionation column is placed between the flask and the condenser.
Fractionating columnsThe mixture on heating forms vapours of both the higher- and lower-boiling liquids. As these vapours go up in the fractionating column; the vapours of the higher-boiling liquid condense as the column provides a surface that is cooler than its boiling point. The vapours of the lower-boiling liquid remain in vapour form and rise up in the column. Thus the lower-boiling liquid is collected as the distillate and the higher boiling liquid is left in the flask.
Some organic liquids have high boiling points but are steam-volatile, i.e. the vapours of such liquids are miscible with steam. When steam is bubbled through such liquids, the liquid comes out as the distillate along with water. This method makes it possible to distil liquids at the temperature of steam (373 K) even though the actual boiling point of the liquid is much more. This method helps to purify steam-volatile compounds from non steam-volatile compounds. Essential oils present in flowers are obtained by this method. Aniline is purified by steam distillation. Another advantage of steam distillation is that the compounds that are unstable around their actual boiling points can be distilled at a much lower temperature without undergoing decomposition.
The principle of steam distillation is based on the fact that a liquid boils when its vapour pressure is equal to the atmospheric pressure, patm. If the vapours of water and those of the organic liquid are miscible, then such a mixture will boil when the total vapour pressure of steam (psteam) and that of the organic liquid (pliquid) is equal to the atmospheric pressure (patm). According to Dalton's law of partial pressures.
The mass of organic compound that distils over during steam distillation depends upon its molar mass and vapour pressure.