Ask A Scientist© Chemistry Archive

   
  >>
 >>    name         Crystal
 >>    status       student
 >>    age          19
\
 >>    Question -   Why do some solids have decreasing solubility as the
 >> temperature
 >>increases? An example is Cesium sulfate.

Basically, any substance that gives off heat when dissolving in watger will
have greater solubility at low termperatures.  Think of Le Chatelier's
Principle - if heat is a product of the reaction, increasing the
temperature will drive the reaction backwards.  Lower solubility at higher
temperature will not always be obvious, because rates of reactions are
increased by temperature.  So, solids will dissolve faster in hot water
than in cold water, even if they don't dissolve as much.  If you don't wait
for the solution to come to equilibrium (saturation), more may dissolve at
high temperature...initially.  So, you have to be sure the solution is
truly saturated.


                Richard Barrans Jr., Ph.D.
                Chemical Separations Group
                Chemistry Division CHM/200
                Argonne National Laboratory
                9700 South Cass Avenue
                Argonne, IL 60439
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