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Hydrocarbons are chemical compounds that contain the elements carbon and hydrogen only. They are compounds that are obtained from the fossil fuel crude oil by a process called fractional distillation.

Fractional distillation

Fractional distillation separates the crude oil mixture into a number of smaller, different parts called fractions. The fractionating column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top.

The crude oil is evaporated and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms, smaller molecules nearer the top and longer molecules nearer the base of the column.

Oil fractions

The diagram below summarises the main fractions from crude oil and their uses and the trends in properties. Note that the gases leave at the top of the column, the liquids condense in the middle and the solids stay at the bottom.

As you go up the fractionating column, the hydrocarbons have:

• lower boiling points
• lower viscosity (they flow more easily)
• higher flammability (they ignite more easily)

This means that in general hydrocarbons with small molecules make better fuels than hydrocarbons with large molecules.