Combustion is a chemical reaction that gives off heat and light. In most cases, combustion involves the rapid combination of oxygen with a fuel to produce burning. The fuel may be a solid, liquid, or gas. Combustion occurs, for example, when oxygen in the air reacts with charcoal in a barbecue grill. When oxygen combines slowly with another substance, the reaction is usually called oxidation. The rusting of iron is an example of oxidation.
In most cases, combustion occurs between a gaseous fuel and the oxygen in the air. The fuel may begin as a solid or liquid, but it must be vaporized (changed to a gas, or vapor) before it can burn. Substances vaporize at their surface. The molecules on the surface are attracted to one another and the molecules underneath. Heat can energize surface molecules of liquids and some solids enough to overcome this attraction. These molecules then escape into the air. The process in which solids vaporize in this way is called sublimation. This process rarely occurs.
Most combustible solids turn into a vapor in a process called pyrolysis or thermal degradation. These solids consist of large molecules whose surface atoms are tightly bonded (joined) to the atoms underneath. Heat destroys bonds at or near the surface, thereby breaking up the molecules. Small molecules or molecular fragments that result can then escape from the surface, thus becoming vapors. In many cases, a solid material called char remains on the surface.
The lowest temperature at which a solid or liquid produces enough vapor for combustion to occur in the presence of an external energy source, such as a spark or flame, is called its flash point. At a higher temperature, called the ignition temperature, a solid or liquid can sustain combustion without an external energy source. The energy given off by burning fuel is called heat of combustion.
Sometimes, a substance suddenly ignites without having contact with a spark or flame. This is called spontaneous combustion. It occurs when chemical reactions within the substance produce heat that cannot escape. Spontaneous combustion may occur when piles of oily rags are left unattended.
