How Does Fire Work? The Chemistry of Combustion
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Fire is not a thing but a process. A burning log is a place where the carbon and hydrogen in the wood are combining with oxygen from the air fast enough to give off light, and the flame is where the reaction happens, in the gas above the wood rather than in the wood itself. Humans have used it for a million years, and the chemistry of what is going on in it was not understood until the 1770s.
Fuel, oxygen and heat
Combustion is a reaction between a fuel and an oxidiser, almost always the oxygen in air, that releases energy. Wood, paper, petrol, natural gas and candle wax are all compounds of carbon and hydrogen, and when they burn completely the carbon becomes carbon dioxide and the hydrogen becomes water vapour, both of them lower in energy than the fuel and oxygen they came from. The difference is released as heat and light.
The reaction does not start by itself, because breaking the bonds in the fuel and the oxygen takes energy before the new bonds can release more. That is why a match is needed. Once a small region is hot enough, the heat it produces raises the neighbouring fuel to the same temperature, and the reaction spreads on its own. Firefighters teach this as the fire triangle: fuel, oxygen and heat, all three needed to start and keep a fire, any one removed to stop it. Water works by taking the heat; a lid on a chip pan takes the oxygen; a firebreak takes the fuel.
The flame is a gas
A solid log does not burn directly. Heated by the match, it decomposes and gives off flammable gases and vapours, and those are what ignite. The flame sits above the wood in the zone where the rising fuel gases meet incoming air, and the charcoal left behind glows rather than flames because it is burning as a solid at its surface, slowly, without the gas stage. A candle shows the process at its simplest: the wick draws liquid wax up, the flame's heat vaporises it, and the vapour burns in a teardrop of flame that never touches the wick.
Inside the flame the chemistry is a chain reaction. The heat breaks fuel and oxygen molecules into fragments called radicals, which are highly reactive and attack other molecules, producing more radicals, so that each step breeds the next. Fire extinguishers filled with certain powders or halogen gases work by mopping up the radicals and breaking the chain, which is why they put out a fire without cooling it or smothering it.
Why it glows, and what the colours mean
The yellow of a candle or a wood fire is not the burning gas. It is soot: tiny particles of carbon formed where the fuel is rich and the oxygen is short, heated to around a thousand degrees and glowing like the filament of an old light bulb. The particles burn away as they reach the edge of the flame, which is why a candle flame gives off no smoke unless it is disturbed. A gas hob burns blue because the gas is mixed with air before it ignites, so there is no soot; the blue is light given off by the reaction's molecular fragments themselves, and it is a hotter, cleaner flame.
Other colours come from metals. Sodium burns yellow, which is why a pinch of salt makes a gas flame flare; copper burns green, strontium red and potassium violet, and those are the colours of fireworks. The temperature of a flame depends on the fuel and on how well it is mixed with oxygen, from about 1,000 degrees in a candle to 1,900 in a gas hob and over 3,000 in a welding torch fed with pure oxygen.
Smoke, ash and incomplete burning
A fire with too little oxygen burns incompletely. Instead of carbon dioxide it makes carbon monoxide, a gas with no smell that binds to haemoglobin more tightly than oxygen does and is the main killer in house fires and in badly ventilated rooms with gas heaters. It also makes smoke, which is unburned soot and tars, and leaves more ash. Ash itself is the part of the fuel that was never combustible: the minerals a tree drew from the soil, which is why wood ash is a fertiliser.
The reverse is also possible. Fuel spread very finely in air, such as flour dust, sawdust or coal dust, burns so fast that it explodes, because every particle has oxygen all round it. Grain silos and flour mills have blown up for this reason, and a fine mist of fuel exploding is exactly what a petrol engine relies on. The essentials of any fire:
- •A fuel containing carbon or hydrogen, or another substance oxygen will react with
- •Oxygen, usually from the air, mixed with the fuel as gas or vapour
- •Enough heat to start the chain of radical reactions
- •The reaction's own heat, keeping the neighbouring fuel above ignition temperature
- •Products: carbon dioxide, water, light, and soot or carbon monoxide if oxygen is short
How it was understood
For most of the eighteenth century chemists believed burning released a substance called phlogiston from the fuel, which explained why wood lost weight when it burned and failed to explain why metals gained weight. Antoine Lavoisier, weighing everything with a precision nobody had matched, showed in the 1770s that burning was combination with a gas from the air, which he named oxygen, and that the total mass was conserved. He was guillotined in 1794 during the French Revolution; the mathematician Lagrange said it took an instant to cut off his head and a century would not produce another like it.
The takeaway
Fire is a self-sustaining chain reaction in which fuel vapour combines with oxygen, releasing enough heat to keep the reaction going and enough light to see it by. It needs fuel, oxygen and heat to start and stops when any is removed; its yellow glow is hot soot, its blue is clean burning, and its smoke and carbon monoxide are the signs of a fire that cannot get enough air.