[aeacfm]

is this true ,a chemical  (monocomponent) in closed vessel and no contact with the environment will it be expired ? if it is possible  for multicomponent chemicals like karl fisher reagent what is the reason for this in the monocomponent chemicals ?

[Stepan]

It depends on chemical and application. Let say:
-high purity analytical solvents accumulate impurities from air, glass, caps and accidental contamination;
-ketones and some other absorb oxygen from air and accumulate peroxides
-salts adsorb CO2 and H2O
-all unsaturated and some saturated organic compound polymerizes

This is less important (with exception of peroxides, which can explode one day) if you work with non-analytical reagents.

Also, "expiration" not always means "dispose". In many occasions you test the chemical and verify (in writing) that it satisfies you need, and then you extend expiration date for the next term.

[aeacfm]

It depends on chemical and application. Let say:
-high purity analytical solvents accumulate impurities from air, glass, caps and accidental contamination;
-salts adsorb CO2 and H2O

ok but with carefully closed reagents and highly pure (ACS grade) what is the source of contamination or impurities 

[aeacfm]

i mean no source of reaction available for the material

[Stepan]

It all depends on application and storage conditions.

World is not perfect and there is no such thing as ABSOLUTELY stable and absolutely "NO CONTACT" with environment. If you use the chemicals, you open them from time to time, and therefore expose them to air and cross-contamination. In my lab (analytical) we assign different shelf life for different chemicals based on their stability and what they are used for. This is derived from our experience and for our lab. For example:

Shelf storage
Hexane 99% - 20 years (You are right it is very stable and nothing happens to it);
Methanol 99% - 5 years (we do not care if it absorbs a little bit of water);
Carbon Disulfide 99.99% - 1 year (it undergoes some kind of reaction and turns yellowish. We also see accumulation of impurities on ppm level
Formaldehyde (37%) - polymerizes

Freezer storage
Dinirtophenyl Hydrazine - 6mth, builds up impurities
MDI - 6 mth polymerization catalysed by traces of moisture
THF - 6 mth - accumulates peroxides (from air even when tightly closed)

[aeacfm]

aha !!!!i get it  many thanks