Fire burns as a reaction when matter changes form and is part of a chemical reaction that produces heat and light. In order for a fire to start, some form of fuel must be heated to its ignition temperature.
After a fuel is heated to its burning point, part of it is decomposed and released as a volatile gas. This gas is known as smoke and contains hydrogen, carbon and oxygen. The remainder forms char, which is mostly carbon and ash and includes any other noncombustible elements. The next part of the burning process requires the volatile gases released to reach a certain temperature, which is about 500 degrees Fahrenheit for wood. Once this temperature has been reached, the molecules in the gas break down and the atoms combine with oxygen to form carbon dioxide, water, carbon monoxide, nitrogen and carbon. This chemical process causes flammable objects to burn. At the same time, the carbon from the char reacts with oxygen to form a much slower reaction. This particular reaction explains how charcoal in a grill can remain hot for long periods of time. A further side effect of these reactions is heat. The heat is what ultimately ends up sustaining a fire until the fuel has been completely burned up.