A chemical reaction is a process in which one or more substances, the reactants, are transformed into another or other substances, the products. A flame is a striking manifestation of combustion, which consists of the intense combination of a substance with oxygen. The reaction with oxygen can be mild such as rust, spontaneously occurring in iron exposed to atmospheric air. The decomposition of a chemical compound into chemical elements, as in the electrolysis of water, or the formation of a chemical compound from its constituents, as in the synthesis of ammonia from nitrogen and hydrogen, are examples of chemical reactions performed in the laboratory. A vessel with several chemical species inside of which chemical reactions take place, is a thermochemical system. Chemical reactions can be endothermic when heat is absorbed by the thermochemical system, or exothermic when heat is released.