Bleaching is the process of whitening materials by sunlight, ultraviolet radiation, or chemicals that reduce or oxidize dyes into a colorless form. Hydrogen peroxide is used to bleach wool, silk, and cotton; hypochlorites, including bleaching powder (see below), are used for cotton; and sodium chlorite for synthetic fibers. Sulfur dioxide bleaches are impermanent. Careful control of pH is essential.
Bleaching powder is a white powder consisting of calcium hypochlorite and basic calcium chloride, made by reacting calcium hydroxide with chlorine. It is used for bleaching and as a disinfectant, but in time loses its strength.