Fuels are substances that react with oxygen to release useful energy (exothermic). Most of the energy is released as heat, but light energy is also released.
About 21 per cent of air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for 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. These are compounds of hydrogen and carbon only. When they burn completely:
In general, for complete combustion:
hydrocarbon + oxygen → carbon dioxide + water
Here is the equation for the complete combustion of methane, used in Bunsen burners:
methane + oxygen → carbon dioxide + water
CH4 + 2O2 → CO2 + 2H2O
Here is the equation for the complete combustion of propane, used in bottled gas:
propane + oxygen → carbon dioxide + water
C3H8 + 5O2 → 3CO2 + 4H2O
Ethanol, a simple alcohol, also combusts to form carbon dioxide and water:
ethanol + oxygen → carbon dioxide + water
C2H5OH + 3O2 → 2CO2 + 3H2O