Chlorine was the first halogen to be separated and recognized as an element. It was named in 1811 by Humphry Davy from a Greek term for its greenish yellow color. Huge deposits of solid salt, mostly sodium chloride, and salts dissolved in the oceans are vast reservoirs of chloride compounds. "Salt" is a general term for a metal and nonmetal combination; there are many different salts. To obtain chlorine an electrical current is applied to brine, a water solution of sodium chloride. Chlorine gas is produced at one electrode. The chlorine must be separated by a membrane from the other electrode, which produces sodium hydroxide.
Chlorine gas itself is toxic. It attacks the respiratory tract and can be fatal. For this reason it was used as a weapon during World War I. Chlorine in solutions has been used as a disinfectant since 1801. It was very effective in hospitals in the 1800s, particularly in an 1831 cholera epidemic in Europe. Chlorine bleaches are employed in most water treatment systems in the United States as well as over much of the rest of the world and in swimming pools.
Chlorine will combine directly with almost all other elements. Large amounts are used yearly for making chlorinated organic compounds, bleaches, and inorganic compounds. Organic compounds, ones which have a skeleton of carbon atoms bonded to each other, can contain halogen atoms connected to the carbon atoms. Low molecular weight organic chlorine compounds are liquids and are good solvents for many purposes. They dissolve starting materials for chemical reactions, and are effective for cleaning such different items as computer parts and clothing ("dry cleaning"). These uses are now being phased out because of problems that the compounds cause in Earth's atmosphere.
Chlorine-containing organic polymers are also widely employed. Polymers are large molecules made of many small units that hook together. One is polyvinyl chloride (PVC), from which plastic pipe and many other plastic products are made. Neoprene is a synthetic rubber made with another chlorine-containing polymer. Neoprene is resistant to the effects of heat, oxidation, and oils, and so is widely used in automobile parts.
Many medicines are organic molecules containing chlorine, and additional chlorine compounds are intermediate steps in the synthesis of a variety of others. Most crop protection chemicals, herbicides, pesticides, and fungicides have chlorine in them. Freon refrigerants are chlorofluorocarbons (CFCs). These perform well because they are volatile, that is they evaporate easily, but they are not flammable. Freon 12, one of the most common, is CCl2F2, two chlorine atoms and two fluorine atoms bonded to a carbon atom.
Chlorine is part of several compounds, such as the insecticide DDT, that are soluble in fats and oils rather than in water. These compounds tend to accumulate in the fatty tissues of biological organisms. Some of these compounds are carcinogens, substances that cause cancer. DDT and other pesticides, polychlorinated biphenyls (PCBs), and dioxins are substances that are no longer manufactured. However, they are still present in the environment, and disposal of materials containing these compounds is a problem.