Bleach is not stable.

excerpt from this web site :

Propri?t?s chimiques !
les eaux et extraits de Javel se d?composent lentement ? temp?rature ambiante avec formation de chlorate et de chlorure de sodium et lib?ration d'oxyg?ne.
3 NaClO --> NaClO3 + 2 NaCl
2 NaClO --> 2 NaCl + O2
La d?composition est acc?l?r?e par la lumi?re, la chaleur et la pr?sence de nombreux m?taux.... Les eaux de javel dilu?es sont beaucoup plus stables que les extraits.

(loosely translated ... or is it loosy translation ??? <img src="/images/graemlins/grin.gif" alt="" />)

Chemical properties :
bleach will slowly degrade at room temperature giving off chlorate, sodium chloride and oxygen (re formulae : NaClO = NaOCl)
Decomposition (or is it "Breaking down" ?) will be faster if light, heat or some metals are present.
Diluted bleach is more stable than extracts.
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another web page, in english
Physical properties

3.3.3 Description
.........
Boiling Point: Decomposes above 40?C (104 deg F)
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Odour chlorine (bleach) odour
Odour Threshold: Not applicable. Odour is due to
breakdown products such as chlorine.
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another one...
sodium hypochlorite
n.
An unstable salt, NaOCl, usually stored in solution and used as a fungicide and an oxidizing bleach.
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web page
Stability:
Slowly decomposes on contact with air. Rate increases with the concentration and temperature. Exposure to sunlight accelerates decomposition. Sodium hypochlorite becomes less toxic with age.
Hazardous Decomposition Products:
Emits toxic fumes of chlorine when heated to decomposition. Sodium oxide at high temperatures.
Hazardous Polymerization:
Will not occur.
Incompatibilities:
Ammonia (chloramine gas may evolve),amines, ammonium salts, aziridine, methanol, phenyl acetonitrile, cellulose, ethyleneimine, oxidizable metals, acids, soaps, and bisulfates.
Conditions to Avoid:
Light, heat, incompatibles.
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