Crude oil, alkanes and fractional distillation

Crude oil is a finite resource formed over millions of years from the remains of ancient plankton buried in mud. It is a mixture of many hydrocarbons: compounds of hydrogen and carbon only. Most are alkanes, a homologous series with the general formula CₙH₂ₙ₊₂: methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. Alkanes are saturated: they have only single C–C bonds.

Fractional distillation separates crude oil into fractions of molecules with similar chain lengths. The oil is heated until it evaporates, and the vapour rises up a column that is hot at the bottom and cooler at the top. Each fraction condenses at a different height. Short chains have low boiling points, low viscosity and are easy to ignite; long chains are thick and less flammable. Complete combustion of an alkane gives carbon dioxide and water.