A bleach is a chemical that removes color or whitens, often via oxidation. Common chemical bleaches include household "chlorine bleach", a solution of approximately 3-6% sodium hypochlorite (NaOCl), and "oxygen bleach", which contains hydrogen peroxide or a peroxide-releasing compound such as sodium perborate or sodium percarbonate. To bleach something is to apply bleach, sometimes as a preliminary step in the process of dyeing. Bleaching powder is calcium hypochlorite.

Many bleaches have strong bactericidal properties, and are used for disinfecting and sterilizing.

Other types of bleaches

Chlorine dioxide is used for the bleaching of wood pulp, fats and oils, cellulose, flour, textiles, beeswax, skin, and in a number of other industries.

In the food industry, some organic peroxides (benzoyl peroxide, etc.) and other agents (e.g. bromates) are used as flour bleaching and maturing agents.

Peracetic acid, ozone and hydrogen peroxide and oxygen are used in bleaching sequences in the pulp industry to produce totally chlorine free (TCF) paper.

Not all bleaches are hazardous and have an oxidizing nature. Sodium dithionite is used as a powerful reducing agent in some bleaching formulas. It is commonly used to bleach wood pulp used to make newsprint.

Hazards

Since most bleaches are strong oxidizing agents, they can be extremely hazardous, especially when reacted with other common household chemicals.

Mixing sodium hypochlorite with acids like vinegar, drain cleaners containing sodium bisulfate (sodium hydrogen sulfate), or even lemon juice can release chlorine gas. Hypochlorite and chlorine are in equilibrium in water; the position of the equilibrium is pH dependent and low pH (acidic) favors chlorine,

Cl₂ + H₂O \rightleftharpoons H⁺ + Cl^- + HClO

Chlorine is a respiratory irritant that attacks mucous membranes and burns the skin. As little as 3.5 ppm can be detected as an odor, and 1000 ppm is likely to be fatal after a few deep breaths. Exposure to chlorine has been limited to 0.5 ppm (8-hour time-weighted average—40 hour week) by OSHA in the U.S.

Sodium hypochlorite and ammonia react to form a number of products, depending on the temperature, concentration, and how they are mixed. . The main reaction is chlorination of ammonia, first giving chloramine (NH₂Cl), then NHCl₂ and finally nitrogen trichloride (NCl₃). These materials are very irritating to eyes and lungs and are toxic above certain concentrations. Lastly there is bleach containing sodium perchlorate.