Combustion is a chemical process whereby rapid oxidation produces heat. Phrased a different way, it is the process that produces heat on a cold evening when a fire is lit in a fireplace. Three things are required for combustion to occur: an initial ignition source, such as a match, a fuel, such as firewood, and an oxidant, which is oxygen. Combustion also results in a number of products. Organic combustion produces carbon dioxide, water and energy.
Combustion is a process whereby chemical bonds are broken and new bonds are formed. Energy is required to break the molecular bonds, meaning this part of the combustion process is endothermic. Energy is produced, or released, when new bonds are formed, meaning this portion of the combustion process is exothermic. If the overall process produces more energy than is used, the sum of the process is exothermic and produces energy as heat or heat and light. If a material produces an exothermic reaction, it is said to be combustible.
As noted, every combustion process requires an initial influx of energy to break the first bonds. This energy is provided in the form of an ignition source, such as a spark or flame. Once the combustion process begins producing energy (exothermic), the combustion process will continue until either the fuel or the oxidant is completely consumed. Once initiated, an exothermic combustion process is self-supporting.
The first required reactant in combustion is a fuel. Many of these fuels, called combustibles, are organic. Organic materials contain carbon, hydrogen and oxygen. However, some inorganic materials, such as magnesium, are also combustible. The second required reactant in combustion is an oxidant. Oxygen is the universal oxidant and is required for all combustion. Combustion will not occur without both of these reactants. Remove the fuel from a fire and it will go out. Likewise, remove the oxidant, by smothering the flames, and the fire will go out as well. This is the purpose behind fire extinguishers.
The combustion of organic materials results in a number of products. The first product of organic combustion is carbon dioxide. The second product of organic combustion is water, typically released as water vapor. The third product of organic combustion is energy, released as heat or heat and light. Because there are other molecules present in most fuels, the combustion process is not entirely clean. This means small amounts of other materials are produced, many of which are potentially harmful. Inorganic combustion does not produce carbon dioxide or water. For example, when magnesium (fuel) reacts with oxygen (oxidant), the result of the combustion process is magnesium oxide and heat. The one constant in combustion, regardless of the fuel, is the release of energy as heat or heat and light.