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Fractional distillation is used to separate crude oil into simpler, more useful fractions. A fraction of crude oil is a group of hydrocarbon molecules of similar size with similar boiling points. Their similar boiling points mean that they can be separated by fractional distillation.

Fractional distillation

During the fractional distillation of crude oil:

• heated crude oil enters a tall fractionating column or tower, which is hot at the bottom and gets cooler towards the top
• vapours from the oil rise through the column
• vapours condense when they become cool enough
• liquids are led out of the column at different heights

Trends in physical and chemical properties

All hydrocarbon molecules have very strong chemical bonds between atoms. They also have a weaker force of attraction between molecules. This is called an intermolecular force. Longer hydrocarbon molecules have a stronger intermolecular force. More energy is needed to move them apart so they have higher boiling points. This makes them less volatile and therefore less flammable.

The longest hydrocarbons have very high boiling points. They leave the column as a hot liquid called bitumen.

Shorter hydrocarbon molecules have weaker intermolecular forces and lower boiling points. They are highly volatile and therefore extremely flammable.

The shortest hydrocarbons have very low boiling points. They do not condense, but leave the column in the gas state.