A bleach is a
chemical that can remove or lighten color, usually via oxidation.
Types of Bleach
There are
several types of bleach. Chlorine bleach usually contains sodium hypochlorite.
Oxygen bleach contains hydrogen peroxide or a peroxide-releasing compound such
as sodium perborate or sodium percarbonate. Bleaching powder is calcium
hypochlorite. Other bleaching agents include sodium persulfate, sodium
perphosphate, sodium persilicate, their ammonium, potassium and lithium
analogs, calcium peroxide, zinc peroxide, sodium peroxide, carbamide peroxide,
chlorine dioxide, bromate, and organic peroxides (e.g., benzoyl peroxide).
While most
bleaches are oxidizing agents, other processes can be used to remove color. For
example, sodium dithionite is a powerful reducing agent that can be used as a
bleach.
How Bleach
Works
An oxidizing
bleach works by breaking the chemical bonds of a chromophore (part of a
molecule that has color). This changes the molecule so that it either has no
color or else reflects color outside the visible spectrum.
A reducing
bleach works by changing the double bonds of a chromophore into single bonds.
This alters the optical properties of the molecule, making it colorless.
In addition to
chemicals, energy can disrupt chemical bonds to bleach out color. For example,
the high energy photons in sunlight (e.g., ultraviolet rays) can disrupt the
bonds in chromophores to decolorize them.
http://chemistry.about.com/od/chemistryfaqs/f/bleach.htm
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