Chlorine was discovered in 1774 by Carl Wilhelm Scheele when he reacted manganese dioxide with hydrochloric acid, then called muriatic acid. Chlorine is a greenish yellow gas at normal temperature and pressure. It is a halogen, and is therefore extremely reactive. It combines with nearly all elements. Chlorine, like the other halogens, is a diatomic element, which means it is always found combined with itself.
Chlorine gas irritates mucous membranes and burns the skin.
A few deep breaths of the gas can result in death, and for that reason chlorine was used as a poison gas during World War I. Chlorine also forms one of the most corrosive acids known, hydrochloric acid.