Chlorine

The most common compound of chlorine, sodium chloride, has been known since ancient times; archaeologists have found evidence that rock salt was used as early as 3000 B.C. The first compound of chlorine ever made by humans was probably hydrochloric acid (hydrogen chloride gas dissolved in water), which was prepared by the Arabian alchemist Rhazes around A.D.900. Around A.D.1200, aqua regia (a mixture of nitric and hydrochloric acids) began to be used to dissolve gold; it is still the only liquid that will dissolve gold. When gold dissolves in aqua regia, chlorine is released along with other nauseating and irritating gases, but probably nobody in the thirteenth century paid much attention to them except to get as far away from them as possible.

The credit for first preparing and studying gaseous chlorine went to Carl W. Scheele (1742-1786) in 1774. Scheele was a Swedish chemist who discovered several other important elements and compounds, including barium, manganese, oxygen, ammonia, and glycerin. Scheele thought that chlorine was a compound, which he called dephlogisticated marine acid air. All gases were called airs at that time, and what we now know as hydrochloric acid was called marine acid because it was made from sea salt. The word dephlogisticated came from a completely false theory that slowed the progress of chemistry for decades and which is best left unexplained.

It was not until 1811 that Sir Humphry Davy (1778-1829) announced to the Royal Society of London that chlorine gas was an element. He suggested the name chlorine because it is the same pale, yellowish green color that sick plants sometimes develop, a color that is known as chloros in Greek. (The sick plants are said to have chlorosis.)