A decomposition reaction is a type of chemical reaction in which a compound is broken down into simpler components. It is the opposite of chemical synthesis, in which elements or relatively simple compounds combine to produce one that is more complex. Since a decomposition reaction involves the breaking of chemical bonds, it requires the addition of energy; this may come from heat, an electrical current or other sources. Sometimes, a catalyst will speed up the reaction or allow it to take place at a lower temperature. These reactions are used industrially in the production of some elements — especially reactive metals — and in the laboratory for the analysis of samples. Thermal decomposition is used in the industrial production of quicklime for cement manufacture and various other purposes. Electrolysis is used in the production of reactive metals. For example, sodium is produced by the electrolysis of molten salt (sodium chloride). This also produces chlorine gas, which has many industrial uses, although most chlorine is produced by the electrolysis of salt solutions in water. Decomposition reactions involving electrolysis are also used to make the extremely reactive element fluorine, and as a “clean” way of generating hydrogen for fuel. There are some scientific applications that depend on decomposition reactions in order to analyze materials. In mass spectrometry, for example, a small sample of the material of interest is split into ions, which are separated according to their charges and masses. The composition of the material can then be determined.