Fuels are substances that react with oxygen to release useful energy. Most of the energy is released as heat, but light energy is also released.

About 21 per cent of the air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for complete combustion.

Complete combustion

Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen.

Fuels such as natural gas and petrol contain hydrocarbons - which are compounds of hydrogen and carbon only. When hydrocarbons burn completely:

  • the carbon oxidises to carbon dioxide
  • the hydrogen oxidises to water (remember that water, H2O, is an oxide of hydrogen)

In general, for complete combustion:

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

Incomplete combustion

Incomplete combustion occurs when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced instead of carbon dioxide.

In general, for incomplete combustion:

hydrocarbon + oxygen → carbon monoxide + carbon + water

The carbon is released as soot. Carbon monoxide is a poisonous gas, which is one reason why complete combustion is preferred to incomplete combustion. Gas fires and boilers must be serviced regularly to ensure they do not produce carbon monoxide.

Here are the equations for the incomplete combustion of propane, where carbon is produced rather than carbon monoxide:

propane + oxygen → carbon + water

C3H8 + 2O2 → 3C + 4H2O