Chlorine is one of the most abundant halogens in the crust. At room temperature it is a yellow-green, highly toxic gas with a density of about 3.2 g/L and a melting point around −102 °C; it almost never enters civilian products in elemental form, but is converted into downstream materials through the chlor-alkali industry. The chlor-alkali process electrolyzes brine, simultaneously yielding chlorine gas, hydrogen, and sodium hydroxide; chlorine then reacts with ethylene to form vinyl chloride monomer, which polymerizes into polyvinyl chloride PVC, one of the world's highest-volume plastics, used in pipes, profiles, windows and doors, cable insulation, flooring, and packaging film. In public health, chlorine and its compounds disinfect tap water, swimming pools, and hospital environments, underpinning modern drinking-water safety. Chlorine also goes into pulp and textile bleaching, pesticide and pharmaceutical synthesis, and refrigerant production. It must be stressed that chlorine itself is highly toxic and corrosive, so transport and use must be strictly sealed.
Sodium chloride NaCl is table salt, both a seasoning and food preservative and the basic feedstock of the chlor-alkali industry, electrolyzed to yield chlorine and caustic soda. Hydrogen chloride HCl dissolves in water to form hydrochloric acid, used for steel pickling, metal surface treatment, oil-well acidizing, and pharmaceuticals. Vinyl chloride monomer VCM polymerizes into polyvinyl chloride PVC, widely used in construction pipes, wire and cable, and artificial leather. Sodium hypochlorite NaClO and bleaching powder Ca(ClO)2 are common chlorine bleaches and disinfectants, used for pulp and fabric bleaching and environmental disinfection. Potassium chlorate KClO3 is a strong oxidant used in match heads, fireworks, and explosives. Methyl chloride CH3Cl is a feedstock for silicones and butyl rubber; phosgene COCl2, though highly toxic, is a key intermediate for polyurethane and polycarbonate synthesis. Ferric chloride FeCl3 is used as a water-treatment flocculant and for etching printed circuit boards.
