Catalytic cracking is the breaking down of long chain hydrocarbon molecules into smaller, more useful molecules using heat and a catalyst.

Crude oil usually undergoes catalytic cracking as part of its refining process.

Vapour containing the unwanted fractions is passed over a very hot catalyst surface and this breaks up the long molecules into smaller fragments, which can then be condensed into useful fuels.

Alternatively, the long chains are mixed with steam and heated to a very high temperature so that thermal decomposition reactions then occur.

These will have shorter chains than the original compounds and hence will have lower boiling points and will burn more easily.