Greenish yellow gas. Poisonous, with a pungent odor. One of the elements categorized in the halogen family. found naturally only in a combined form, like table salt (sodium chloride). One of the most reactive substances. Made from the electrolysis of brine. It is considered to be an essential disinfectant used in water purification and as a bleaching agent used in paper and textile industry. Its presence causes depletion of the ozone layer in upper atmosphere mainly as a component of chlorinated organic compounds. It was discovered in 1774 by the Swedish chemist, Carl Wilhelm Scheele (1742-86).