Re: Why does a supersaturated solution solidify if solute is added?

First, let's establish what makes a solution "saturated" or 
"supersaturated."

A solvent can only hold so much of a given solute in solution.  When that
level is reached, the solution is said to be "saturated," and any 
remaining solute will be left on the bottom of the container.

More solute can be forced into solution by heating the mixture.  As the
mixture is heated, its capacity to dissolve the solute increases.  The
solution remains close to the saturation point as it is heated, and more
of the solute is dissolved.

When the solution is cooled, the solute may begin to come out of 
solution.  The solution at this point is still saturated, because as it
cools, the solute that it can no longer contain will simply fall out of
solution.  Thus, a solution with stuff sitting on the bottom is 
"saturated."  It can't really be supersaturated, because the remaining
stuff on the bottom will more than likely cause the excess solute to
come out of solution.

OK.

If the solution cools past its saturation point, but no solute precipitates
out, the solution is said to be "supersaturated."  It contains more solute
than it ought to be able to at a given temperature.  The reason that a 
solution becomes supersaturated is complicated, but in general terms, 
there has been no opportunity for the solute to begin to crystallize.
You can imagine the solution just *bursting* with energy, with the solute
molecules desperate to line themselves up so that they can crystallize
out of solution.  They just need a little bit of energy, a little extra
"umph" to do so.  That can be provided by dropping a "seed crystal" in
(it gives the solute molecules something to crystallize upon), or by
scratching the surface of the glass (also gives the solute a surface to
crystallize on), or sometimes even by just shaking the mixture (gives the
molecules the energy they need in the form of kinetic energy).

So, a clear solution that contains more solute than it ought to is
supersaturated.  Adding a tiny crystal gives the solute the opportunity
to solidify, and *bam*, the excess falls out of solution.  Only the
excess amount will come out, though -- you will only get crystals forming
until the point that the solution is saturated.