Describe the process of fractional distillation of crude oil and explain how the properties of hydrocarbon fractions are related to the size of their molecules.

1. Crude oil is heated in a furnace until it vapourises.
2. The vapours rise into a fractionating column where they are cooled.
3. As the vapours travel up the column they condense at different levels, each level corresponding to a specific temperature.
4. The condensates are collected as separate fractions – for example, naphtha, kerosene, diesel and heavy oil.
5. Each fraction contains hydrocarbons with a similar number of carbon atoms; shorter‑chain molecules have lower boiling points and are more volatile, while longer‑chain molecules have higher boiling points, higher viscosity and are less flammable.