Crude oil is a finite resource found in rocks. It was formed over millions of years from the remains of tiny, ancient sea animals and plants, mainly plankton, that were buried under mud.

It is a mixture of many different compounds that boil at different temperatures. The crude oil mixture can be separated to make useful fuels.

You can separate a mixture of liquids by distillation. Simple distillation of crude oil can produce liquids that boil within different temperature ranges. These liquid mixtures are called fractions.

Most compounds in crude oil are hydrocarbons. This means that their molecules contain only hydrogen and carbon.

Many of these hydrocarbons are alkanes, with the general formula CnH(2n+2) for alkanes.

The first four alkane molecules are: CH4 (methane), C2H6 (ethane), C3H8 (propane), C4H10 (butane)

All carbon-carbon bonds are single covalent bonds. This means that hey contain as many hydrogen atoms as possible in each molecule.

Crude oil is separated into fractions at refineries using fractional distillation. This can be done because the boiling point of a hydrocarbon depends on the size of its molecule. The higher the molecule, the higher the boiling point of the hydrocarbon.

1- The crude oil is vaporised first and then put inside the fractionating column.

2- In the fractionating column the crude rises up the column.

3- In the fractionating column there is a temperature gradient. Long chain hydrocarbons with a high boiling point condense near the bottom. Short chain hydrocarbons with a lower boiling point rises up the column and condenses.

Fractions with smaller hydrocarbon molecules and lower boiling temperatures have lower viscosities so they are runny liquids. They are very flammable so they ignite easily. They also burn with clean flames, producing little smoke. This makes them useful as fuels.