Crude oil has to be separated into fractions to be useful. How is this done?

Crude oil has lots of molecules with different numbers of carbon atoms in them. A fractional distillation column is used to separate the molecules. It is hottest at the bottom and coolest at the top. Bitumen is distilled at the bottom, and refined gas comes out the top. This is to do with different boiling points of the various molecules in the crude oil. Molecules with more carbon atoms such as bitumen have a higher boiling point, and so need more (heat) energy to overcome the intermolecular forces between molecules. So, the smaller molecules (with fewer carbon atoms) will boil first and therefore rise to the top of the column where they can exit as gas. Larger molecules will be boiled last and therefore stay at the bottom of the column where they can exit.