Combustion reactions

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.

A triangle showing the elements needed for a fire: oxygen, fuel and heat.A fire needs a fuel, oxygen (or air), and heat

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.

FireHow to put it outPart removed
Chip pan (oil) fireCover the pan with a damp clothOxygen
Forest fireMake a fire break (cut down a line of trees)Fuel
Forest fireSpray with waterHeat

Complete combustion

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:

  • the hydrogen atoms combine with oxygen to make water vapour, H2O
  • the carbon atoms combine with oxygen to make carbon dioxide, CO2
  • the maximum amount of energy is released

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.

An inverted glass funnel sits over a lit candle. The stem of the funnel connects to a U-tube which rests in an ice bath. Water will condense in the U-tube. The other end of the U-tube connects to a test tube of limewater, which is also connected to a water pump.Investigating combustion

The carbon dioxide produced can be detected using limewater. This turns milky (cloudy white) when carbon dioxide is bubbled through it.

Three test tubes used in carbon dioxide test
The limewater test for carbon dioxide

Incomplete combustion

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:

  • carbon monoxide, CO, a colourless toxic gas
  • particles of carbon, which appear as soot and smoke, and which cause breathing problems