Combustion

Combustion, or burning, is a high-temperature exothermic chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized products in a mixture called smoke. A flame appears only when substances undergoing combustion vaporize, and the heat it releases can make the reaction self-sustaining once activation energy has started it. Combustion was the first controlled chemical reaction discovered by humans and remains the primary method humanity uses to produce energy, whether from fossil fuels such as coal and oil or from renewable fuels such as firewood, for uses including cooking, electricity generation, and industrial or domestic heating. Complete combustion is difficult to achieve in practice, so real combustion often produces unburned or partially oxidized byproducts such as carbon monoxide and nitrogen oxides.