Fractional distillation can be used to separate a mixture of compounds with different boiling points such as crude oil or air.
Air is a mixture of nitrogen, oxygen, carbon dioxide and some other gases. The composition of air is shown in the pie chart below.
To separate this mixture the air must be cooled to -200 oC, which turns it into a liquid. It is then warmed up slowly and each of the liquids boil at a different temperature and can be collected and removed.
Fractional distillation can also be used to separate crude oil into the many compounds that it is made up of. This is very important as crude oil contains some very useful compounds such as petrol and diesel. The products of fractional distillation are shown in the diagram below: