More about fractions

Properties of fractions

Each crude oil fraction contains a mixture of hydrocarbons. The hydrocarbons in a fraction are mostly hydrocarbons called alkanes. They have similar (but not identical):

For example, the gases fraction contains hydrocarbons with one to four carbon atoms. These:

  • have boiling points below room temperature
  • are very flammable
  • have a low viscosity

The hydrocarbons in different fractions differ in these properties. For example, the bitumen fraction contains hydrocarbons with more than 35 carbon atoms. These have:

  • boiling points well above room temperature
  • are very difficult to ignite
  • have a high viscosity

Hydrocarbon fuels can undergo complete combustion or incomplete combustion, depending on the amount of oxygen available.

Complete combustion

Complete combustion of a hydrocarbon fuel happens when there is a good supply of air. Carbon and hydrogen atoms in the fuel react with oxygen and are oxidised in an exothermic reaction:

In a Bunsen burner, this occurs when the air hole is fully open.

Sections of a Bunsen burner blue flameA Bunsen burner blue flame

In general:

hydrocarbon + oxygen → carbon dioxide + water

Here are the equations for the complete combustion of propane, used in bottled gas:

propane + oxygen → carbon dioxide + water

C3H8 + 5O2 → 3CO2 + 4H2O


Write a word and balanced symbol equation for the complete combustion of methane, CH4, found in natural gas.

Methane + oxygen → carbon dioxide + water

CH4 + 2O2 → CO2 + 2H2O