Crude oil, also known as petroleum, is the main source for most carbon-containing compounds. It is a mixture of hydrocarbons, substances which contains only carbon and hydrogen. The majority of hydrocarbons found in crude oil are alkanes which consist of carbon atoms bound by single bonds only.

The hydrocarbons found in crude oil have different uses but, as a mix, have no use at all. Therefore, they need to be separated, a feat achieved through the process of fractional distillation. These products are usually then converted into even more useful hydrocarbons through the process of cracking.

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Fractional distillation

Hydrocarbons can be separated due to the fact that they possess different boiling points. This is because of chain length: the more carbon atoms in a chain the greater the Van der Waal’s forces and the higher the intermolecular attraction.

Make sure you remember for your A level chemistry revision guide; The process of fractional distillation goes as follows:

• The crude oil is put through a tall tower known as a fractioning column. The bottom section is very hot while the top section is much colder.
• The crude oil starts near the bottom where most of it boils and begins to rise up the column.
• As it rises up it cools down and condenses back into a liquid which can then be tapped off. This is composed of the larger hydrocarbons with higher boiling points.
• Smaller hydrocarbons condense later as they have lower boiling points and are tapped off higher up the column.

For your A level Chemistry revision guide; only intermolecular forces are broken leaving the molecules themselves in tact. However, the process does not separate the crude oil into components of pure hydrocarbons but fractions composed of hydrocarbons that possess similar boiling points. In a lot of cases this mix can be used, as shown in the table below. However, sometimes purer components are needed.