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.
Fire as a record of the Earth's oxygen
Fire is rarer in the universe than it looks from a campsite. It needs fuel and free oxygen together, and Earth is the only planet known to have both: plenty of dry plant material and an atmosphere rich in oxygen, which exists only because photosynthesising organisms have been releasing it for billions of years. Before land plants spread, there was nothing on land to burn. The oldest fossil charcoal, the clearest evidence of ancient wildfire, dates from around 420 million years ago, soon after the first plants had colonised the land.
Because fire depends so strongly on oxygen, geologists use charcoal in rocks as a rough gauge of the past atmosphere. Laboratory burning experiments show that plant material struggles to keep burning when the oxygen level falls well below today's 21 percent, and burns much more fiercely when it is higher. Periods with little fossil charcoal suggest lower oxygen, while the coal swamps of the Carboniferous, when oxygen is thought to have been higher than now, contain abundant charcoal from frequent fires. A burnt twig preserved in rock is therefore evidence about the air ancient animals breathed as well as about a fire.
Fire without gravity
The familiar teardrop shape of a flame is made by gravity. Hot combustion gases are lighter than the air around them, so they rise, pulling fresh air in at the base and stretching the flame upward. In orbit there is no up for hot gas to rise towards. A candle there burns as a small, dim, roughly spherical flame, often blue, because oxygen can reach it only by slowly diffusing in and the products can leave only by diffusing out. Less soot forms in these conditions, so the bright yellow glow of a candle on Earth largely disappears.
Space experiments have also revealed something new about fire itself. In 2012, experiments on the International Space Station ignited tiny droplets of the fuel heptane, and some kept burning after their visible flame had gone out. They had switched to what chemists call a cool flame, a slow, faint form of combustion at a few hundred degrees, driven by a different set of chain reactions from the ones in an ordinary flame. Cool flames had been known on Earth since the nineteenth century, but they are fleeting there, and studying them steadily in orbit matters for designing cleaner, more efficient engines.
What happens when you strike a match
A safety match keeps its key chemicals apart until the moment of use. The head contains potassium chlorate, which releases oxygen when heated, along with a fuel such as sulphur and a binding glue. The striking strip on the box contains red phosphorus and powdered glass. Drawing the head across the strip creates enough frictional heat to convert a tiny amount of red phosphorus into the far more reactive white form, which ignites in air and sets off the chlorate and sulphur. The flame then passes to the wooden stick, which is often treated with a fire retardant so that it does not keep glowing after being blown out.
Earlier matches put white phosphorus in the head itself, so they could be struck on any rough surface, and they came at a terrible cost. Workers who dipped the heads breathed in phosphorus fumes, and many developed phossy jaw, a disease in which the jawbone rotted away. In 1888 the women and girls at the Bryant and May factory in east London went on strike over pay and conditions, a landmark protest in trade union history. Safety matches, developed in Sweden in the mid-nineteenth century, made it possible to use harmless red phosphorus instead, and an international convention of 1906 moved to ban white phosphorus matches.
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.