Complete and incomplete 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:
• Carbon oxidises to carbon dioxide
• Hydrogen oxidises to water - Water, H₂O, is an oxide of hydrogen

General reaction for complete combustion:

• Hydrogen + Oxygen → Carbon dioxide + Water
• 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.

General reaction for incomplete combustion:

• Hydrogen + Oxygen → Carbon monoxide + Carbon + Water
• 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.