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Chlorination is the process by which the element chlorine reacts with some other substance. Chlorination is a very important chemical reaction both in pure research and in the preparation of commercially important chemical products. For example, the reaction between chlorine and methane gas produces one or more chlorinated derivatives, the best known of which are trichloromethane (chloroform) and tetrachloromethane (carbon tetrachloride). The chlorinated hydrocarbons constitute one of the most commercially useful chemical families, albeit a family surrounded by a myriad of social, political, economic, and ethical issues. One member of that family, as an example, is dichlorodiphenyltrichloroethane (DDT). Although one of the most valuable pesticides ever developed, DDT is now banned in most parts of the world because of its deleterious effects on the environment.

The term chlorination is perhaps best known among laypersons in connection with its use in the purification of water supplies. Chlorine is widely popular for this application because of its ability to kill bacteria and other disease-causing organisms at relatively low concentrations and with little risk to humans. In many facilities, chlorine gas is pumped directly into water until it reaches a concentration of about one ppm (part per million). The exact concentration depends on the original purity of the water supply. In other facilities, chlorine is added to water in the form of a solid compound such as calcium or sodium hypochlorite. Both of these compounds react with water releasing free chlorine. Both methods of chlorination are so inexpensive that nearly every public water purification system in the world has adopted one or the other as its primary means of destroying diseasecausing organisms.

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