What is combustion?
Combustion is a chemical reaction in which a fuel reacts with oxygen to release heat and usually light. It is often just called burning. Three things are needed for combustion, shown as the fire triangle: fuel, oxygen (from air) and heat (to reach the ignition temperature). Remove any one of them and the fire goes out.
Key idea
fuel + oxygen → carbon dioxide + water + heat (for fuels that contain carbon and hydrogen). Fire needs fuel + oxygen + heat.
Complete and incomplete combustion
When there is plenty of oxygen, a fuel undergoes complete combustion. It burns with a clean blue flame and produces mainly carbon dioxide (CO₂) and water. When oxygen is limited, incomplete combustion happens. It gives a yellow smoky flame and can produce carbon (soot) and poisonous carbon monoxide (CO).
Example
A gas stove with the air holes open burns with a blue flame (complete combustion). If the holes are blocked, the flame turns yellow and sooty (incomplete combustion).
Putting out fires
Firefighters break the fire triangle: water cools the fuel (removes heat), a blanket or CO₂ cuts off oxygen, and clearing away material removes the fuel.
- Carbon monoxide is dangerous because it is colourless and odourless.
- Good ventilation gives enough oxygen for complete combustion.