What Is Combustion?
Combustion is a chemical reaction between a fuel and oxygen that releases energy, usually as heat and light. It is what happens when something burns. Combustion is an important type of oxidation reaction.
For combustion to occur, three things must be present at the same time — known as the fire triangle:
- Fuel — the substance that burns
- Oxygen — (usually from the air)
- Heat — to start the reaction (ignition temperature)
Remove any one side of the fire triangle and combustion stops. This is the basis of firefighting.
Complete Combustion
When there is plenty of oxygen available, a hydrocarbon fuel (containing carbon and hydrogen) undergoes complete combustion, producing:
- Carbon dioxide (CO₂)
- Water (H₂O)
The word equation for the complete combustion of methane (natural gas):
methane + oxygen → carbon dioxide + water
The symbol equation: CH₄ + 2O₂ → CO₂ + 2H₂O
Complete combustion produces a blue flame and releases the maximum amount of energy from the fuel.
You are using combustion every time you ride in a car. The petrol (a hydrocarbon mixture) burns with oxygen from the air in the engine. The products — CO₂ and H₂O — are expelled through the exhaust pipe. This is also why cars contribute to rising CO₂ levels in the atmosphere.
Incomplete Combustion
When there is limited oxygen, incomplete combustion occurs instead. Products include:
- Carbon monoxide (CO) — a toxic, colourless, odourless gas
- Soot (carbon particles, C) — solid black particles
- Water (H₂O) — still produced
Incomplete combustion produces an orange or yellow flame and releases less energy. Carbon monoxide is dangerous because it binds to haemoglobin in red blood cells, preventing oxygen transport.
Carbon monoxide (CO) is a silent, colourless, odourless killer. Every year, people die from CO poisoning caused by faulty gas appliances burning with insufficient oxygen. Carbon monoxide detectors in homes save lives — they should be installed near gas boilers, fireplaces, and in bedrooms.
Testing for Combustion Products
| Product | Test | Positive result |
|---|---|---|
| Carbon dioxide (CO₂) | Bubble gas through limewater | Limewater turns milky/cloudy |
| Water (H₂O) | Hold cold glass or cobalt chloride paper in flame | Glass fogs up; blue cobalt paper turns pink |
Uses of Combustion
Combustion is used to:
- Generate electricity in power stations (burning coal, gas, or oil)
- Power engines (petrol, diesel, jet fuel)
- Heat homes (gas boilers, wood fires)
- Cook food (gas cookers)
Real world: Car engines and petrol combustion
A petrol car engine is a combustion machine. Petrol (a mixture of hydrocarbons) is injected into cylinders and ignited by a spark, reacting with air. The rapid, complete combustion releases energy that pushes pistons, moving the car. The products — CO₂ and H₂O — exit through the exhaust. This is why cars emit CO₂ and contribute to climate change.
Real world: Gas cookers and the blue flame
A gas cooker burns natural gas (mostly methane, CH₄) with plenty of air, producing a clean blue flame — the signature of complete combustion. The products are CO₂ and H₂O vapour. If the flame turns orange or yellow, it signals incomplete combustion (less air reaching the burner), producing sooty carbon particles and dangerous carbon monoxide. A well-maintained cooker keeps the flame blue.
Real world: Carbon monoxide detectors in homes
Faulty gas boilers, blocked flues, or gas appliances burning in poorly ventilated rooms can produce carbon monoxide (CO) from incomplete combustion. CO is odourless and colourless — you cannot detect it without a sensor. CO binds to haemoglobin in red blood cells 200 times more strongly than oxygen, preventing oxygen transport and causing death within hours. Carbon monoxide detectors save lives by sensing CO before it reaches dangerous levels.
Where do you see this in real life? Car exhausts (combustion of petrol), gas cooker flames (complete = blue, incomplete = orange), candles and fireplaces (incomplete combustion = soot and smoke), CO detectors in kitchens — combustion is the most common chemical reaction in everyday life.
Practice Problems
A candle burns with an orange, sooty flame. What does this tell you about the type of combustion and the oxygen supply?
Show answer
Write the word equation for the complete combustion of propane (a hydrocarbon fuel).