Combustion must occur in the presence of O2, which is a molecule that consists of two oxygen atoms. The process of combustion often results in the emission of water and carbon dioxide. An example of combustion would be lighting a match. When you apply friction by striking a match against a matchbox, the molecules on the tip of the match react with the O2 molecules in the air, causing a combustion reaction that forms a flame.
Combustion is called an "exothermic reaction" because it emits heat. Most combustion reactions occur when an organic compound or hydrocarbons react with oxygen molecules. Many organic molecules and hydrocarbons contain carbon and hydrogen atoms, which is why they burn easily. Methane, gasoline, kerosene, propane, butane, methyl alcohol and ethyl alcohol are all examples of hydrocarbons that combust easily, which is why many of these are used as fuels.
Combustion is often used to fuel machines because it converts molecules into energy through heat transfer. The objective in converting these chemicals into energy is to achieve what is called a stoichiometric combustion, which is where all of the fuel's energy is used. Stoichiometric combustion is nearly impossible to achieve, but it is the objective of many engineers when designing engines, power plants or any other machine that converts energy and uses combustion.
Combustion always produces heat, light and water as byproducts, but it can also produce numerous other hazardous byproducts. These include carbon dioxide, carbon monoxide, nitric oxide, nitrogen dioxide, sulfur dioxide, soot and ash. In cars, power plants and houses, the U.S. Environmental Protection Agency regulates the emission of these byproducts by requiring that certain standards and regulations are followed when dealing with combustion waste. For example, cars have filters that reduce the amount of carbon dioxide that is emitted into the air.