Organic compounds are the compounds that most biological substances are made out of. They are made up primarily of carbon.

Hydrocarbons are a common type of organic compound and they are made up on hydrogen (hydro-) and carbon (-carbon) atoms.

The main source of hydrocarbons is crude oil. This is the oil that is extracted from oil fields.

Hydrocarbons are come in many different sizes and these have different uses.

Crude oil separates these hydrocarbons into their different sizes through fractional distillation.

Works by heating crude oil and separating the different hydrocarbons by their boiling points.

This works because it relies on dispersion forces and these depend on the size of the molecule.

The smaller molecules have a lower boiling point and the larger molecules have higher boiling points.

See the simulation below for a demonstration of fractional distillation. Observe how the smaller hydrocarbons rise up and the bigger hydrocarbons settle below.