Burning, which chemists call combustion, is a reaction with oxygen. When a hydrocarbon burns in plenty of air, its carbon atoms end up in carbon dioxide and its hydrogen atoms end up in water.
Natural gas is mostly methane, and it burns on gas stoves and in many home boilers. Each methane molecule reacts with two oxygen molecules, O₂, from the air.
Counting atoms
The 2 in front of O₂ means two oxygen molecules, which hold 4 oxygen atoms. The 2 in front of H₂O means two water molecules, with 4 hydrogen atoms and 2 oxygen atoms between them. Count each kind of atom, and both sides match.
| Atom | CH₄ + 2 O₂ | CO₂ + 2 H₂O |
|---|---|---|
| carbon | 1 | 1 |
| hydrogen | 4 | 2 × 2 = 4 |
| oxygen | 2 × 2 = 4 | 2 + 2 = 4 |
Why burning gives out heat
Burning one methane molecule breaks 4 C–H bonds and 2 O=O bonds. It makes 2 C=O bonds in carbon dioxide and 4 O–H bonds in water. Together the new bonds are stronger, so making them gives out more energy than breaking the old ones took in. The extra energy comes out as heat and light: burning is exothermic.
Burning needs a start. A gas stove needs a spark or a flame to break the first bonds. After that, the heat from the burning gas breaks bonds in the next molecules, so the flame keeps itself going.
Petrol, paraffin candle wax and the propane in barbecue gas bottles are hydrocarbons too, and they burn to carbon dioxide and water in the same way. The carbon dioxide from burning coal, oil and gas builds up in the air, and it is the main cause of climate change.