Chlorine

Category: halogen Atomic number: 17 Colour: yellowy green Melting point: −102°C (−151°F) Boiling point: −34°C (−29°F) First identified: 1774

You will be very familiar with chlorine in the everyday form of salt (or sodium chloride), which is a crucial part of our nutritional needs as part of the foods we eat. However, as with other elements, chlorine has more than one face: when the German army used chlorine gas as a weapon at Flanders in 1915, it resulted in about 5,000 deaths and many more suffered terribly from its effects.

Chlorine is not found in its pure form in nature – it was first isolated in 1774 by Swedish chemist Carl Wilhelm Scheele, who heated hydrochloric acid with manganese dioxide. The result was a yellowy-green gas with a foul, choking smell, which could be dissolved in water to give an acid solution. Scheele didn’t believe that the gas he had produced was pure, but in 1807 Humphry Davy carried out further investigations and announced the existence of a new element (which was named after the Greek word for yellowy green).

One compound of chlorine is PVC (polyvinyl chloride), that versatile plastic used for everything from window frames to blood bags in hospitals. It is also used widely in the pharmaceutical industry to trigger chemical reactions. But chlorine is probably best known for its role as a disinfectant, as it kills bacteria; it is present in many household bleaches and is used to make tap water and swimming pools safe. The latter use originated in London, following a cholera outbreak, when the pioneering doctor John Snow realized that an infected well in Soho was the cause. He attempted to disinfect the pump from the well with chlorine. For the rest of the century there were various isolated cases of this kind of use of the chemical, before comprehensive chlorination of drinking water started to be adopted in Europe and America early in this century. Snow also experimented with the chlorine compound chloroform and used it while helping Queen Victoria to give birth.

Our attitudes to some chlorine products have changed significantly over time. Chloroform and the dry-cleaning solvent, carbon tetrachloride, were in common use in the past, but are now carefully controlled as they can damage the liver. At one stage, chlorofluorocarbons were also widely used, especially in aerosols – better known by the acronym CFCs; they were implicated in the ongoing destruction of the ozone layer. Since the 1980s there has been a huge global reduction in their use, which has happily helped the ozone layer to stabilize over recent years.