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
  • they 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 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 balanced equation for the complete combustion of methane, CH4, found in natural gas.

CH4 + 2O2 → CO2 + 2H2O

Incomplete combustion

Incomplete combustion happens when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced. Less energy is released than during complete combustion.

In a Bunsen burner, incomplete combustion occurs when the air hole is closed.

Bunsen burner yellow flameA Bunsen burner yellow flame

You will learn more about incomplete combustion in Polluting the atmosphere.