Definition · Plain-language
Combustion
Combustion is a rapid exothermic reaction between a fuel and oxygen that releases heat and usually light.
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A fuel reacting rapidly with oxygen
Combustion is the chemical name for burning: a fuel reacts quickly with an oxidising agent, almost always the oxygen in air, releasing energy as heat and usually light. The reaction is exothermic, meaning it gives out more energy than it takes in, which is what makes a flame hot. For combustion to begin and continue, three things are needed together — fuel, oxygen and enough heat to start the reaction — often pictured as the fire triangle. Remove any one and the burning stops.
Complete and incomplete combustion
When a hydrocarbon fuel such as methane burns in plenty of oxygen, it undergoes complete combustion, producing only carbon dioxide and water and releasing the maximum energy. When oxygen is limited, combustion is incomplete: as well as carbon dioxide and water, it produces carbon monoxide — a colourless, toxic gas — and carbon as soot. Incomplete combustion releases less energy and is the reason faulty heaters and stoves can be dangerous. A yellow, smoky flame often signals incomplete combustion, while a clean blue flame signals complete combustion.
Where combustion appears
Combustion is everywhere in daily life and industry. Burning natural gas, petrol, diesel, coal and wood releases energy to heat homes, generate electricity and power engines. Respiration in living cells is a slow, controlled form of oxidation that releases energy in a related way, though biologists usually treat it separately. Rapid, uncontrolled combustion is fire; an extremely fast combustion that produces a pressure wave is an explosion. Because most fuels contain carbon, combustion is also the main human source of carbon dioxide emissions.
Key facts
At a glance
- Definition: a rapid reaction between a fuel and oxygen releasing heat
- Energy: exothermic — gives out heat and usually light
- Needs: fuel, oxygen and heat (the fire triangle)
- Complete: produces carbon dioxide and water
- Incomplete: also produces carbon monoxide and soot
- Example: methane + oxygen → carbon dioxide + water
Common misconceptions
What people often get wrong
Often heard: Combustion can happen without oxygen or another oxidiser.
Actually: Combustion requires an oxidising agent, almost always oxygen. Remove the oxygen and burning stops, which is why smothering a fire puts it out. Some special oxidisers other than oxygen exist, but an oxidiser is always needed.
Often heard: All combustion produces only carbon dioxide and water.
Actually: That is complete combustion. When oxygen is limited, incomplete combustion also produces toxic carbon monoxide and soot, and releases less energy. The products depend on how much oxygen is available.
Often heard: Combustion and explosion are entirely different reactions.
Actually: An explosion is essentially a very rapid combustion that releases energy and gas so fast it creates a pressure wave. The chemistry is the same kind of fuel–oxygen reaction; the difference is the speed and confinement.
