Fire is the visible effect of the process of combustion – a special type of chemical reaction. It occurs between oxygen in the air and some sort of fuel. The products from the chemical reaction are completely different from the starting material.

The fuel must be heated to its ignition temperature for combustion to occur. The reaction will keep going as long as there is enough heat, fuel and oxygen. This is known as the fire triangle.

Combustion is when fuel reacts with oxygen to release heat energy. Combustion can be slow or fast depending on the amount of oxygen available. Combustion that results in a flame is very fast and is called burning. Combustion can only occur between gases.

Chemical reaction in the combustion process

Fuels can be solids, liquids or gases. During the chemical reaction that produces fire, fuel is heated to such an extent that (if not already a gas) it releases gases from its surface.

Only gases can react in combustion. Gases are made up of molecules (groups of atoms). When these gases are hot enough, the molecules in the gases break apart and fragments of molecules rejoin with oxygen from the air to make new product molecules – water molecules (H 2 O) and carbon dioxide molecules (CO 2 ) – and other products if burning is not complete.

The heat generated by the reaction is what sustains the fire. The heat of the flame will keep remaining fuel at ignition temperature. The flame ignites gases being emitted, and the fire spreads. As long as there is enough fuel and oxygen, the fire keeps burning.

Fuel + oxygen (from the air) = combustion products (mainly CO 2 + H 2 O) + heat energy.

Complete combustion

In complete combustion, the burning fuel will produce only water and carbon dioxide (no smoke or other products). The flame is typically blue. For this to happen, there needs to be enough oxygen to combine completely with the fuel gas.

Many of us use methane gas (CH 4 ), commonly known as natural gas, at home for cooking. When the gas is heated (by a flame or spark) and if there is enough oxygen in the atmosphere, the molecules will break apart and reform totally as water and carbon dioxide.

CH 4 (g) + 2O 2 (g) + heat → CO 2 (g) + 2H 2 O(g) + heat methane + oxygen + heat → carbon dioxide + water + heat

Incomplete combustion

If there is not enough oxygen available during a chemical reaction, incomplete combustion occurs, and products such as carbon (C) and carbon monoxide (CO) as well as water and carbon dioxide are produced. Less heat energy is released during incomplete combustion than complete combustion.

In incomplete combustion, the burning flame is typically yellow or orange and there is smoke.