Combustion is another name for burning. It is an example of an exothermic reaction, a reaction that releases energy to the surroundings. This is mostly thermal energy, but light energy and sound energy are also released. Note that some other reactions are endothermic reactions – they take in energy from their surroundings.
The fire triangle shows the three things needed for a fire to start and keep going.
If one of the sides of the fire triangle is removed, a fire will not start, and a fire that is already burning will go out. Fire-fighting relies on this principle. The fire will go out when the fuel runs out, but it is often unsafe to leave a fire that long. Different types of fires need to be tackled in different ways.
|Fire||How to put it out||Part removed|
|Chip pan (oil) fire||Cover the pan with a damp cloth||Oxygen|
|Forest fire||Make a fire break (cut down a line of trees)||Fuel|
|Forest fire||Spray with water||Heat|
Coal, oil and natural gas are fuels that are widely used. They contain hydrocarbons, which are compounds of hydrogen and carbon only. When the fuel burns, its hydrocarbons react with oxygen. If there is plenty of air, complete combustion happens:
Natural gas is mostly methane, CH4. Here are the equations that model its complete combustion:
methane + oxygen → water + carbon dioxide
CH4 + 2O2 → 2H2O + CO2
Candles are made from hydrocarbons. The diagram shows how they can be used in the laboratory to investigate combustion.
The carbon dioxide produced can be detected using limewater. This turns milky (cloudy white) when carbon dioxide is bubbled through it.
If there is not enough air or oxygen for complete combustion, incomplete combustion happens instead. Water vapour and carbon dioxide are still produced, but two other products are also produced: