Combustion and Flame

Combustion is a chemical process in which a substance reacts with oxygen to produce heat (and usually light). The substance that burns is called a combustible substance or a fuel — for example wood, coal, kerosene, LPG and paper. Substances that do not burn, such as stone, glass and iron, are non-combustible.

For combustion to take place, three conditions must all be met. Together they are called the fire triangle:

• A combustible substance (fuel).
• A supporter of combustion — usually the oxygen in air, which helps things burn.
• Reaching the ignition temperature — the lowest temperature at which a substance catches fire.

If any one of these three is removed, burning stops. That is the science behind putting out fires: we either remove the fuel, cut off the oxygen, or cool the substance below its ignition temperature. A substance cannot catch fire, however much we heat it, unless it reaches its ignition temperature — this is why a matchstick lights easily but a piece of coal needs much more heating.

Substances that have a very low ignition temperature and can easily catch fire with a flame are called inflammable (or flammable) substances — for example petrol, alcohol, LPG and dry grass. These must be stored and handled with great care, away from flames.

Combustion is of different types, depending on how fast it happens:

• Rapid combustion — the substance burns quickly and produces heat and light all at once when a flame is brought near it. Example: burning of LPG when the gas stove is lit.
• Spontaneous combustion — the substance bursts into flames on its own, without bringing any flame near it. Example: phosphorus catching fire in air at room temperature; forest fires sometimes start this way.
• Explosion — a sudden reaction that releases heat, light, sound and a large amount of gas with great force. Example: the bursting of a firecracker.

The combustion of some substances also produces a flame, while others (like coal or charcoal) glow red without a flame. A flame is produced only when the substance gives off gases (vapours) while burning — it is these hot gases that burn to form the flame.

A candle flame has three different zones, which you can see by their colours:

• The innermost zone (around the wick) is dark/black. Here there is unburnt wax vapour and very little oxygen, so it is the least hot.
• The middle zone is yellow and bright. Here the wax burns partly (incomplete combustion), giving light; it is moderately hot.
• The outermost zone is blue and almost non-luminous. Here there is plenty of oxygen, so the wax burns completely — this is the hottest part of the flame. This is why goldsmiths use the outer part of a flame to melt metals.

A good fuel should be cheap, easy to store and transport, burn safely without leaving much ash, and give a lot of heat. The amount of heat produced by completely burning 1 kilogram of a fuel is called its calorific value, measured in kilojoules per kilogram (kJ/kg). A fuel with a higher calorific value gives more heat per kilogram.

Burning fuels can be harmful: incomplete combustion produces poisonous carbon monoxide; carbon dioxide and other gases cause global warming and acid rain; and unburnt particles cause air pollution and breathing problems. To control a fire, firefighters remove one side of the fire triangle — water cools the substance and cuts off air (but is not used on electrical or oil fires), while a carbon dioxide (CO₂) extinguisher blankets the fire and cuts off oxygen, and is safe for electrical and oil fires.