A common type of chlorine utilized for water treatment is chlorine gas. The addition of chlorine gas to water in a treatment process produces hypochlorous acid (HClO), a known oxidizer and disinfectant. Hypochlorous acid in turn, will also quickly dissociate into hypochlorite ions and hydrogen ions.
At 99.9% concentration, chlorine gas is the most concentrated and the least expensive form of the substance. Denser than air, it immediately sinks to the ground once released from its container. Another property of chlorine gas is that it is toxic and it is this toxicity that makes it effective as a disinfecting agent.
Depending on the quality of the water to be treated, about 1 to 16 mg of chlorine gas is needed per liter of water. Disinfecting water of poor quality requires a higher concentration of chlorine gas or a longer contact time.