Crude oil is one of the most important natural resources. It is finite, which means it will eventually run out, and non-renewable as it cannot be made again. It is a fossil fuel, like coal and natural gas.
Fossil fuels were made over millions of years from the remains of dead organisms that were compressed together, while trapped in the Earth. Dead animals made oil and dead plants made coal.
Crude oil is a mixture of hydrocarbons (substances made of carbon and hydrogen only). In order to separate it into its components it needs to be heated through a process called fractional distillation. Here is a diagram that shows how crude oil is separated:
The component hydrocarbon molecules are long chains held together by intermolecular forces. The forces are broken while boiling.
Lighter molecules with shorter chains need less energy for the forces to be broken, so they are easily separated. They have low boiling points. An example is petrol.
Heavy molecules, like bitumen, have longer chains and more energy is needed to separate them. They have high boiling points.
As the heat in the fractional distillation tower increases, small molecules evaporate first and rise to the top, whereas heavier ones stay at the bottom.
More and more petrol is needed for the increasing requirements of humans, so a substance called paraffin is 'cracked' (broken down) into petrol, with the aid of high temperature and a catalyst (a chemical that speeds up the reaction but is not itself used in it).