Combustion is a chemical reaction in which complex molecules are broken down into smaller, more stable molecules through a rearrangement of atomic bonds. A major component of the chemistry of high-temperature combustion involves radical reactions. However, it is possible to consider combustion as a single overall reaction. Example: H₃C-CH₂-CH₃ + 5O₂ → 3CO₂ + 4H₂O. Carbon dioxide and water are more stable than oxygen and propane. Combustion is an oxidation-reduction reaction, meaning oxidization of a combustible by an oxidizer; combustible is being oxidized during combustion, it is a reducer as it loses electrons; Oxidizer is the part being reduced; it is an oxidizer as it gains electrons. As with any chemical reaction, a catalyst encourages combustion and as it has a high activation energy level, the use of a catalyst enables working at lower temperature. This leads to more complete combustion as in the catalyst of the exhaust of a car, where catalytic metals burn residues contained in the exhaust smoke at lower temperature than in the engine. Concerning solid combustible, the activation energy allows for vaporization or pyrolysis of the combustible. Gas produced will then mix with an oxidizer resulting in a combustible mixture. If the energy produced by the combustion is higher or equal to the quantity of energy required for the combustion, the reaction is then self-sustainable