What makes things burn and which fuels are best
Combustion is a chemical reaction in which a substance reacts with oxygen to give heat and usually light. Three things are needed for a fire: a fuel, oxygen from air, and a temperature at least as high as the fuel’s ignition temperature. Substances with low ignition temperatures, like petrol and LPG, catch fire easily and are called inflammable. Combustion can be rapid, like a gas stove, spontaneous, like phosphorus burning in air, or explosive, like fireworks.
A candle flame has three zones: the outer zone is the hottest because combustion is complete, the middle zone is luminous with partly burnt carbon, and the innermost zone is the least hot with unburnt wax vapour. A good fuel is cheap, easily available, burns at a moderate rate, leaves little residue and has a high calorific value, the heat produced per kilogram, measured in kJ/kg. Burning fuels can release carbon monoxide, soot and sulphur dioxide, which cause pollution and acid rain.
Why it matters
Fire extinguishers work by removing one of the three needs of fire, and water must never be used on electrical or oil fires. Choosing cleaner fuels such as CNG and LPG over wood and coal reduces smoke and protects health. Knowing about carbon monoxide explains why charcoal stoves should not be used in closed rooms.
Key terms
- Combustion
- A chemical reaction in which a substance reacts with oxygen to release heat and light.
- Ignition temperature
- The lowest temperature at which a substance catches fire.
- Calorific value
- The amount of heat produced when one kilogram of a fuel burns completely.
- Inflammable substance
- A substance with a low ignition temperature that catches fire easily.