What is combustion?

Combustion is fast burning: a fuel joins with oxygen and gives out heat and light. The fuels we burn most are made of carbon and hydrogen. When they burn, every carbon atom ends up in carbon dioxide (CO₂) and every hydrogen atom in water (H₂O).

For methane, the gas in a kitchen pipeline: CH₄ + 2 O₂ → CO₂ + 2 H₂O. Count them: one carbon, four hydrogens and four oxygens on each side. Burning only swaps partners. No atom is made and none is destroyed, so the mass can't change either. That's the law of conservation of mass: in a closed system, grams in = grams out.

So why does a candle seem to shrink away to nothing? Because in the open, its products are invisible gases that float off, and the oxygen it takes comes from the air. Seal it in a jar and the balance doesn't budge. Antoine and Marie-Anne Lavoisier proved this with careful sealed-vessel experiments in the 1770s and 1780s.

Each fuel carries a different amount of energy: about 50 MJ per kg for methane, 46 for LPG, 44 for petrol and only 16 to 18 for dry wood, which already contains a lot of oxygen.

The gas hob. A 14.2 kg LPG cylinder holds mostly butane, C₄H₁₀. Burn it all and you make about 43 kg of carbon dioxide and 22 kg of water vapour. How can 14.2 kg become 65 kg? Every carbon atom picks up two oxygen atoms from your kitchen air, and oxygen is heavy. The cylinder also used up about 51 kg of oxygen, from roughly 220 kg of air.

That's why a kitchen needs a chimney or an exhaust fan (see ChimneyClear). An hour on a big burner adds about half a kilogram of CO₂ and a quarter of a litre of water to the room. With the windows shut and no hood, CO₂ can climb to several thousand ppm, enough to feel stuffy and sleepy.

Blue or yellow? A blue flame gets enough air mixed in before it burns, so the carbon burns right through. Starve it and specks of carbon, soot, glow yellow, blacken the pan and some carbon monoxide escapes.

Iron that gets heavier. Light a pad of steel wool on a balance and it sinks: the iron joins oxygen to make iron oxide, which weighs more. Rusting is the same reaction, only slow. This was Lavoisier's big clue: burning doesn't release something, it takes in oxygen.

A car or motorbike engine is a combustion chamber that burns a tiny puff of petrol and air thousands of times a minute (see CarClear and MotorcycleClear). The hot gas pushes the piston down, and the burnt gas leaves through the exhaust.

Petrol is made of carbon and hydrogen, roughly CH₁.₈₇. To burn each gram completely it needs about 14.7 grams of air. That's the stoichiometric ratio, and engine computers aim for it all the time, using an oxygen sensor in the exhaust.

Now weigh it. A litre of petrol is only 0.74 kg, but it makes about 2.3 kg of CO₂ and 0.9 kg of water. The carbon picks up oxygen from nearly 11 kg of air. Most of what comes out of the tailpipe is actually nitrogen from that air, passing straight through.

Rich (too little air, λ below 1) leaves carbon monoxide and unburnt fuel (HC). Lean (spare air) runs hot and makes NOx, from nitrogen and oxygen joining in the heat. The catalytic converter is a honeycomb coated with platinum, palladium and rhodium. At λ = 1 it turns CO and HC into CO₂ and water, and NOx back into nitrogen. It can't remove the CO₂, though: that is the fuel's carbon, and it has nowhere else to go.

The fire triangle. A fire needs three things at once: fuel, heat and oxygen. Take away any one and it goes out. Water takes away heat: each kilogram soaks up about 2.6 MJ as it warms and boils into steam. CO₂ from an extinguisher pushes the oxygen out: below about 15 % oxygen (air has 21 %) most flames die. A fire blanket or a lid on a burning pan does the same by cutting off the air.

Never throw water on burning oil. The water sinks, flashes into steam in an instant and throws the burning oil everywhere. Put a lid on it and turn the gas off.

Your body burns too, slowly. Your cells combine food with the oxygen you breathe: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O, exactly the same reaction as burning sugar in a flame. The energy comes out in small steps, as chemical energy your cells can use, and as body heat, not as fire.

So where does fat go when you lose weight? Chemists Ruben Meerman and Andrew Brown followed every atom in 2014: about 84 % leaves as CO₂ through your lungs, and 16 % as water. Your lungs are the main exit for lost weight (see LungsClear). Breathing harder doesn't help, though: only the energy you actually use burns fat.

The vanishing candle. A candle on a balance gets lighter all evening, so it looks as if burning destroys matter. It doesn't: it's an open system. The wax leaves as invisible CO₂ and water vapour, and those gases weigh more than the wax, because they carry oxygen from the air.

Myth-buster: phlogiston. In the 1700s chemists said burning things give off phlogiston, a “fire substance”. A candle getting lighter seemed to prove it. But burnt metals get heavier, so phlogiston would need negative weight. The Lavoisiers weighed everything in sealed vessels and showed that burning takes in oxygen. Phlogiston was never real.

E = mc². Here's the honest fine print. When anything gives out energy, it loses a tiny bit of mass: Δm = E ÷ c². For burning, that is about one part in 10 billion, far too small for any balance to see. Chemistry conserves mass to better than we can measure; nuclear fission changes about 0.1 % of the mass, which is why it's millions of times more powerful.

The silent killer. With too little air, carbon burns only halfway, to carbon monoxide. CO has no smell or colour, and it grabs your blood's haemoglobin more than 200 times as tightly as oxygen. Every winter, people in India die after sleeping with a charcoal angithi or a gas heater in a closed room. Never burn anything in a shut room, and never sleep with it.