Re: Solution and temperature

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

Richard H. wants to know:
> ....
>   What is the actual mechanism by which a solution of sugar and water 
> (for example) holds more solute at higher temperatures?  ....

What happens is that the higher temperature aids a variety of mechanisms
which serve to increase the solvation rate and to decrease (somewhat) the
precipitation rate.  This shifts things so that the equilibrium is restored
only when the concentration of the solute in the solvant is high enough to
increase the precipitation rate enough to match the raised solvation rate.

Some of the enhanced solvation effects are: 1. The higher temperature gives 
the solvent particles more kinetic energy on the average which makes their
collisions with the surface of the solid solute substance more effective in
dislodging the solute particles from the solid and giving them the needed
"escape energy" necessary to get away from the forces which bind them to the
rest of the solid.  2. The higher temperature also gives the solute particles
on the surface more energy to begin with so that they require less extra
energy to be removed from the surface (the activation free energy of solvation
is lowered).  3. The higher temperature causes a (slight) increase
in the collision rate with the surface.  There are probably other effects
involving increased residence time of contact between the solvant particles
and the surface solute particles, etc..  The precipitation rate is decreased
at higher temperatures because the probability of a collision (of a solute
particle from the solution with the solid surface) resulting in a solute
particle becoming attached to the solid is reduced.  This is because the
incoming particle is more likely to have too high of an energy to stick and
will probably just bounce off the surface.  Since the surface is also composed
of more vigorously vibrating particles, the incoming particle needs a narrower
range of lower energies for a high likelihood of sticking--a range that it is
less likely to be in.

David Bowman
dbowman@gtc.gerogetown.ky.us