For other uses, see Bleach (disambiguation).

In chemistry, to bleach something generally means to whiten it or oxidize it. A bleach is a chemical that can produce these effects. Common chemical bleaches include sodium hypochlorite, or "chlorine bleach," and "oxygen bleach," which contains hydrogen peroxide or a peroxide-releasing compound (eg. sodium perborate, or sodium percarbonate). "Bleaching powder" is calcium hypochlorite. Bleaching can be a preliminary step in the process of dyeing.

A bleaching agent, or bleach, is any compound that bleaches the colour out of fabrics, paper, or other materials. Household bleach or sodium hypochlorite is used in the home for whitening clothes, removing stains, and disinfecting. This is because sodium hypochlorite yields chlorine radicals—oxidizing agents readily reacting with many substances.

Hair bleach contains H₂O₂ (hydrogen peroxide), which gives off oxygen radicals as it decomposes. Oxygen and chlorine radicals both have comparable bleaching effects.

Chlorine bleach is often used with laundry detergent and is also commonly used as a disinfectant by homemakers and janitors. Mixing bleach and cleaners containing ammonia can create toxic chloramine gases and an explosive called nitrogen trichloride.

Not all bleaches have to be of oxidizing nature. Sodium dithionite is used as a powerful reducing agent in some bleaching formulas.

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

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

Photography

In most color negative processes, the silver halide crystals present in the emulsion are removed using chemical bleaches (unlike black and white negatives, which contain silver in all the dark areas). Photographic bleach is usually potassium ferricyanide.

Hazards

A problem with chlorine is that it reacts with organic material to form trihalomethanes like chloroform, which is a well known carcinogen. But the benefit of using chlorine to kill the germs in drinking water far outweighs any risk from the tiny trace of chloroform in treated drinking water.

Chlorine is a respiratory irritant. It also attacks mucus 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 should not exceed 0.5 ppm (8-hour time-weighted average - 40 hour week).

Chlorine from typical CFCs like trichlorofluoromethane, which is stable, but reaches the ozone layer, is one of the two radicals formed there: the highly reactive chlorine atom, much more than the dichloromethyl radical, initiates the ozone degradation chain reaction.

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