Crude oil is separated by fractional distillation. Explain how the fractionating column separates crude oil into fractions, and describe two properties of shorter-chain hydrocarbons compared with longer-chain hydrocarbons.

Crude oil is heated in a furnace so that the hydrocarbons evaporate. The vapour rises through a tall fractionating column that is packed with material to give a large surface area. As the vapour ascends it cools; hydrocarbons with lower boiling points condense first at the top of the column, while those with higher boiling points condense further down. Each tray or level therefore collects a fraction that contains molecules with a similar number of carbon atoms. Short‑chain hydrocarbons have lower boiling points than longer‑chain hydrocarbons and are therefore found in the upper fractions. They are also more flammable and less viscous (runnier) than the longer‑chain hydrocarbons.