Bleach
A bleach is a chemical that removes color or whitens, often via oxidation. Common chemical bleaches include "chlorine bleach", a solution of sodium hypochlorite (NaClO), 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.

# Other types of bleach
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 (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 have to be of 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 and concerns
Since bleaches are strong oxidizing agents, they can be quite hazardous, especially when reacted with other common household chemicals.
Mixing sodium hypochlorite with acids like vinegar or drain cleaners containing sodium hydrogen sulfate (sodium bisulfate), or even lemon juice can release chlorine. Hypochlorite and chlorine are in equilibrium in water, the position of the equilibrium is pH dependant 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 odour, 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 US.
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.
NH3 + NaOCl   --> NaOH + NH2Cl
NH2Cl + NaOCl --> NaOH + NHCl2
NHCl2 + NaOCl --> NaOH + NCl3
Additional reactions produce hydrazine, in a variation of the Olin Raschig process.
NH3 + NH2Cl + NaOH --> N2H4 + NaCl + H2O
The hydrazine generated can further react with the monochloramine in an exothermic reaction:
2 NH2Cl + N2H4 --> 2 NH4Cl + N2
Industrial bleaching agents can also be sources of concern.  For example, the use of elemental chlorine in the bleaching of wood pulp produces organochlorines, persistent organic pollutants, including dioxins. However, the use of chlorine dioxide in these processes has reduced the dioxin generation to under detectable levels.

# History
Chlorine was first characterized by the Swedish chemist Carl Wilhelm Scheele in 1774 (as an adherent of the Phlogiston theory, he called it "dephlogisticated marine acid"). French chemist Claude Louis Berthollet, noting the bleaching properties of chlorine, invented hypochlorite bleach in 1789. In French, bleach is known as Eau de Javel, after the village where it was manufactured.
Several alternatives to bleach have recently appeared in industrialized countries.  These substances are touted as being less toxic, and the use of bleach as a stain remover has become less popular in the United States.  However, due to the recent upsurge of illness due to methicillin-resistant Staphylococcus aureus (known as MRSA) and other bacterial pathogens susceptible to bleach, the bleach industry has recovered somewhat, and the use of bleach as a disinfectant is increasing in a variety of industrial and commercial, as well as household settings.

# Chemistry
The process of bleaching can be summarised in the following set of chemical reactions:
Cl2(aq) + H2O(l) \rightleftharpoons H+(aq) + Cl-(aq) + HClO(aq)
The H+ ion of the hypochlorous acid then dissolves into solution, and so the final result is effectively:
Cl2(aq) + H2O(l) \rightleftharpoons 2H+(aq) + Cl-(aq) + ClO-(aq)

# How bleaches work
Color in most dyes and pigments is produced by molecules, such as beta carotene, which contain chromophores.  Chemical bleaches work in one of two ways:
- An oxidizing bleach works by breaking the chemical bonds that make up the chromophore.  This changes the molecule into a different substance that either does not contain a chromophore, or contains a chromophore that does not absorb visible light.
- A reducing bleach works by converting double bonds in the chromophore into single bonds.  This eliminates the ability of the chromophore to absorb visible light.
Sunlight acts as a bleach through a process leading to similar results: high energy photons of light, often in the violet or ultraviolet range, can disrupt the bonds in the chromophore, rendering the resulting substance colorless. Extended exposure often leads to massive discoloration usually reducing the colors to white light and typically very faded blue spectrums.