Hypochlorous acid, formed when chlorine dissolves in water, is antimicrobial. This is the basis for the use of chlorine and halazone in water purification. Organic matter binds chlorine, thus preventing antimicrobial actions. In a given water sample, this process is referred to as the chlorine demand because the chlorine-binding capacity of the organic material must be exceeded before bacterial killing is accomplished. Many preparations of chlorine for water purification do not eradicate all bacteria or entamoeba cysts