Chlorine

Chlorine is a yellow-green poisonous gas with a sharp smell, and at the same time one of the most sought-after products of the chemical industry. It is also part of table salt, without which a person does not live. The difference between an element and a simple substance is unusually vivid here: the chloride ion in salt is harmless, molecular chlorine is lethal.

The name comes from the Greek word for yellow-green.

Free, almost nowhere: chlorine is too reactive. Bound, chiefly as chlorides — rock salt NaCl, sylvite KCl, carnallite. The chloride ion together with sodium accounts for the salinity of the ocean.

Industrially, chlorine is made by electrolysing a solution of sodium chloride; hydrogen and sodium hydroxide are produced alongside it, and all three products are wanted, which makes the process economically neat.

In 1774 Carl Scheele obtained a yellow-green gas by treating the mineral pyrolusite with hydrochloric acid. He took it for a compound — hydrochloric acid combined with oxygen — which fitted the ideas of the day.

Humphry Davy established that chlorine is an element in 1810, showing that it contains no oxygen and cannot be broken into anything simpler. The name is his.

Chlorine is a halogen: it is one electron short of a filled shell, and it takes that electron from almost anything. Its electronegativity is high, exceeded only by fluorine, oxygen and nitrogen.

With metals it reacts directly and often with ignition; with hydrogen in daylight, explosively. In water it partly dissolves and disproportionates: some becomes chloride, some hypochlorous acid, HClO — and it is that acid, not chlorine itself, which does the disinfecting and the bleaching.

Chlorine's oxidation states run from −1 to +7, giving a series of acids from hydrochloric, HCl, to perchloric, HClO₄, and the corresponding salts.

The largest share goes into polymers, above all polyvinyl chloride. Then solvents, intermediates in organic synthesis, and pulp bleaching.

Disinfecting drinking water is an application that has probably saved more lives than any other achievement of twentieth-century chemistry: mass chlorination of water supplies effectively ended cholera and typhoid epidemics in cities.

Organochlorine chemistry has a heavier side. Chlorofluorocarbons, once regarded as ideally safe refrigerants, turned out to destroy the ozone layer and were banned by the Montreal Protocol. Persistent organochlorine pesticides such as DDT accumulate in food chains.

Chlorine was also the first chemical weapon used on a large scale, at Ypres in 1915.

Chloride ions are the principal anions of extracellular fluid; they help maintain osmotic pressure and acid–base balance. The hydrochloric acid of gastric juice, needed for digestion, is built from chloride ions too.

An adult's daily requirement is met by a few grams of table salt.