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Fuels from crude oil

Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels. Unfortunately, there are environmental consequences when fossil fuels such as crude oil and its products are used.

Hydrocarbons and alkanes


Most of the compounds in crude oil are hydrocarbons. This means that they only contain hydrogen and carbon atoms, joined together by chemical bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.


The alkanes are a family of hydrocarbons that share the same general formula. This is:


The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6. Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.

Structure of alkanes

alkaneformulachemical structureball-and-stick model
methaneCH4H - C - H, with an H above and below the carbon atom and four hydrogen atoms
ethaneC2H6two C's and six H'stwo carbon atoms and six hydrogen atoms
propaneC3H8three C's and eight H'sthree carbon atoms and eight hydrogen atoms
butaneC4H10four C's and ten H's atomsfour carbon atoms and ten hydrogen atoms

Notice that the molecular models on the right show that the bonds are not really at 90º.

Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air, which we call burning or combustion.

Boiling point and state at room temperature

Hydrocarbons have different boiling points, and can be either solid, liquid or gas at room temperature:

  • Small hydrocarbons with only a few carbon atoms have low boiling points and are gases.
  • Hydrocarbons with between five and 12 carbon atoms are usually liquids.
  • Large hydrocarbons with many carbon atoms have high boiling points and are solids.

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