The second of the halogens, chlorine is a highly reactive, yellow-green gas. Because of its reactivity, chlorine compounds are quite common, with the best known of these being sodium chloride, an important nutrient that provides the body with chlorine for the production of hydrochloric acid, a strong compound used to break down food in the stomach. Pure chlorine, however, is highly toxic and has been used as a chemical weapon in warfare. But perhaps its most well known use is as a disinfectant for water, where it contributes to the characteristic smell of swimming pools.