Chemistry of Fire

Oxidation is a chemical process in which a substance combines with oxygen. During this process energy is given off, usually in the form of heat. The rusting of iron and the rotting of wood are common examples of slow oxidation. Fire, or combustion, is a rapid oxidation. The burning substance combines with oxygen at a very high rate. Energy is given off in the form of heat and light. Due to the rapid energy production we can feel the heat and see the light as flames.

All matter exists in one of three states – solid, liquid or gas (vapour). The atoms or molecules of a solid are packed closely together, and those of a liquid are packed loosely. The molecules of a vapour are not packed together at all: they are free to move about. In order for a substance to oxidize, its molecules must be surrounded by oxygen molecules. The molecules of solids and liquids are too tightly packed to be surrounded. Thus, only vapours can burn. However, when a solid or liquid is heated, its molecules move about rapidly. If enough heat is applied, some molecules break away from the surface to form a vapour just above the surface. This vapour can now mix with oxygen. If there is enough heat to raise the vapour to its ignition temperature, and if there is enough oxygen present, the vapour will oxidize rapidly – it will start to burn.

Burning is the rapid oxidation of millions of vapour molecules. The molecules oxidize by breaking apart into individual atoms and recombining with oxygen into new molecules. It is during the breaking-recombining process that energy is released as heat and light. The heat that is released is radiant heat, which is pure energy. It is the same sort of energy that the sun radiates and that we feel as heat. It radiates, or travels, in all directions. Thus, part of it moves back to the seat of the fire, to the burning solid or liquid (the fuel). The heat that radiates back to the fuel is called the radiation feedback. Part of this heat releases more vapour, and part of it raises the vapour to the ignition temperature. At the same time, air is drawn into the area where the flames and vapour meet. The result is that the newly formed vapour begins to burn. The flame increases.

This is the start of chain reaction. The burning vapour produces heat which releases and ignites more vapour. The additional vapour burns, producing more heat, which releases and ignites still more vapour. This produces still more heat, vapour and combustion, and so on. As long as there is plenty of fuel available, the fire continues to grow, and more flame is produced.

After a time, the amount of vapour released from the fuel reaches a maximum rate and begins to level off producing a steady rate of burning. This usually continues until most of the fuel has been consumed. Then there is less vapour to oxidize, and less heat is produced. Now the process begins to break down. The less vapour is released, the less heat and flame are produced. So the fire begins to die out. A solid fuel may leave an ash residue and continue to smoulder for some time. A liquid fuel usually burns up completely.

Apart from solid and liquid fuels, there are, of course, flammable gases. Gases burn more intensely than solids and liquids, because they are already in the vapour state. All the radiation feedback goes on igniting the vapour. Gases burn without smouldering or leaving residues. The size and intensity of a gas fire depend on the amount of fuel available.

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