It is used in the separation of homogeneous mixtures when the mix components are liquid. Fractional distillation is based on the different boiling points of the mixture components.
The technique and apparatus used for fractional distillation is the same as for simple distillation, only one thermometer should be placed in the distillation flask, so that the distillation of the lowest boiling liquid can be known. The distillation of the lowest boiling liquid will end when the temperature rises rapidly again.
Fractional distillation is used in the separation of petroleum components. Oil is an oily substance, less dense than water, formed by a mixture of substances. Crude oil is extracted from the earth's crust subsoil and may be mixed with salt water, sand and clay. By decantation the salt water is separated by filtration with sand and clay. After this treatment, the oil is subjected to fractionation to separate its components by fractional distillation. The main fractions obtained in the distillation of oil are: gas fraction, in which the cooking gas is found; fraction of gasoline and benzine; fraction of diesel oil and lubricating oils, and wastes such as petroleum jelly, asphalt and fish.
Fractional distillation is also used to separate the components of a gas mixture. First, the gas mixture must be liquefied by decreasing the temperature and increasing the pressure. After liquefaction, the mixture is subjected to fractional distillation: the lowest boiling gas returns to the gaseous state. This process is used to separate oxygen from atmospheric air, which consists of approximately 79% nitrogen and 20% oxygen and 1% other gases. In the case of this mixture the lowest boiling gas is nitrogen.