Forces Between Molecules
In a hydrocarbon molecule there are: - Strong covalent bonds - Weak intermolecular forces
The intermolecular forces between longer hydrocarbons are stronger than the forces between shorter hydrocarbons.
When a liquid hydrocarbon is boiled, its molecules move faster and faster until all the intermolecular forces are broken and it becomes a gas.
Small molecules have very weak forces of attraction between them and are easy to overcome by heating.
It's the differences in their boiling points which allow us to separate a mixture of hydrocrabons (e.g. crude oil) by the process of fractional distillation.
I HOPE YOU