Crude oil is made of organisms that lived and died millions of years ago – mainly plankton that was buried in mud. It is a complex mixture of hydrocarbons . The carbon atoms are joined together in chains and rings.
Crude oil is an important source of fuels such as petrol, kerosene, heavy fuel oil and liquefied petroleum gas. It is also used as feedstock – the raw material – for the petrochemical industry, which makes plastic, rubber, solvents, lubricants and detergents.
Fractional distillation separates crude oil into simpler, more useful mixtures, because different hydrocarbons have different boiling points. Each simpler mixture is called a fraction.
During fractional distillation:
Small hydrocarbon molecules have weak intermolecular forces, so they have low boiling points. They do not condense, but leave the top of the column as gases.
Long hydrocarbon molecules have stronger intermolecular forces, so they have higher boiling points. They leave the bottom of the column as hot, liquid bitumen.