Crude oil contains many hydrocarbons with different chain lengths. Describe how the properties of hydrocarbon molecules change as chain length increases, and explain how these properties affect the usefulness of longer-chain fractions as fuels.

Longer‑chain hydrocarbons have higher boiling points because the increased surface area allows more London dispersion forces, so they are harder to vapourise.
They are more viscous, i.e. they flow less easily, because the larger molecules have greater intermolecular attraction.
They are less flammable – the larger molecules are less likely to ignite and burn rapidly.
Since fuel performance relies on easy vapourisation and rapid combustion, longer‑chain fractions are less useful as fuels compared with shorter‑chain fractions.
Cracking can be used to break down these long chains into shorter, more useful alkanes and alkenes, which have lower boiling points and higher flammability.