well i was just wondering about this topic for quite some time now but couldn't make any odds out of it. just think : what and why do we have the concept of stability? commonly said an unstable substance would disintegrate or undergo some changes to attain a state of stability... like any ion or radical would react any reagent available to form some product. now just imagine that a during the course of some reaction some unreacted unstable intermediate is formed which couldn't react with anything as the other reagents are exhausted and the whole system is isolated... what would happen to it? it would exist in that unstable state forever( as long we keep it isolated) because it has nothing around it to interact with... so what happens then?

unstable intermediates tend to self react (ie. spontanteously disintigrate) rather than require the presence of another reagent.

 

though the lifetime of unstable and meta stable intermediates tends to be on the order of nanoseconds (its less than this but it helps you get the point without me having to explain everything).

 

you can however capture these unstable states by freezing them in an ultra cold(near 0K) matrix of xenon atoms IIRC. i can't remember exactly how it's done but i've read about it being used to keep helium ionic compounds around long enough to be observed.