Fractional Distillation

  • Crude oil as a mixture isn’t a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are useful, with each fraction having different applications.
  • Each fraction consists of groups of hydrocarbons of similar chain lengths.
  • The fractions in petroleum are separated from each other in a process called fractional distillation.
  • The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain.
  • The size and length of each hydrocarbon molecule determines in which fraction it will be separated into.
  • The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains.
  • Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them.

Fractional Distillation of Crude Oil

Fractional-Distillation, IGCSE & GCSE Chemistry revision notes

Diagram showing the process of fractional distillation to separate crude oil in a fractionating column

  • Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top.
  • Crude oil enters the fractionating column and is heated so vapours rise.
  • Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column.
  • Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off.
  • The different fractions condense at different heights according to their boiling points and are tapped off as liquids.
  • The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases.
  • The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column.


You need to know the names and corresponding uses of each fraction obtained from crude oil.

  • The petrochemical industry is hugely important for modern society and development.
  • The fuels that are used in most modern methods of transport (cars, trains, airplanes etc.) are all based on oil products.
  • Polymers, lubricants, solvents, detergents and adhesives are all products that are obtained from crude oil.
  • The array of fractions in crude oil and the huge range of compounds we can produce from them all stem from carbon’s ability to form homologous series of compounds.
  • The main fractions and their uses are described in the following table.

Petrochemicals Table, IGCSE & GCSE Chemistry revision notes

AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.