An exothermic oxidation reaction that may occur with any organic compound, as well as with certain elements, e.g., hydrogen, sulfur, phosphorus, magnesium. The end products of elemental combustion are oxides; end products of organic compounds are carbon dioxide and water. Examples are (1) for an element: 2H2 + O2 → 2H2O; (b) for an organic compound (carbohydrate): C6H12O6 + 6O2 → 6CO2 + 6H2O. Here, combustion is the reverse of photosynthesis. The heat of combustion is due to rupture of chemical bonds and formation of new compounds. Substances differ greatly in their combustibility, i.e., in their ignition points (solids and gases) or their flash points (liquids). Carbon disulfide burns almost explosively at 100C, whereas rubber hydrocarbon and nylon are difficult to ignite at any temperature. Oxygen is not combustible but actively supports combustion; no oxygen is needed if another oxidizing agent is present, as in the combustion of a mixture of hydrogen and chlorine to form hydrogen chloride. Spontaneous combustion may occur at or even below room temperature (1) by exposure to air of substances that are highly sensitive to oxidation (e.g., phosphorus); (2) by heat buildup from bacterial activity (compost, sewage sludge) or oxidation catalyzed by moisture, as in wet waste materials (paper, cotton, wool); (3) by internal heat accumulation caused by autoxidation (fish oils, linseed oil).See Oxidation; Pyrophoric Material; Autoignition Point; Flash Point.
