Ask A Scientist© Chemistry Archive

name         Bito L.
status       student
age          13

Question -   When you add water to Calcium Chloride and Potassium
Carbonate you'll get a Calcium Chloride and Potassium Carbonate solution.
When you put the 2 solutions together you get a result of solid on the
bottom and liquid on the top. Why does the Potassium Chloride solution
dissolve and the Calcium Carbonate stays as a solid?
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It is all a matter of whether the different components prefer to pack in to a
solid lattice or to be dissolved in water.  Calcium chloride and potassium
carbonate both are able to dissolve in water quite well.  This is because
their crystal lattices are not stable enough to resist the ability of water
to surround and stabilize ions.  The same is true for potassium chloride.
Calcium carbonate, however, forms a nice stable lattice.  The calcium and
carbonate ions prefer being in the lattice to being disoolved in water, so
the solid will form when calcium and carbonate are present.

Actually, it is not an either/or matter.  All ionic substances can be
characterized by a solubility or a solubility product constant, which tells
the extent to which an ionic solid will dissociate (dissolve) in water.
Some are very soluble (sodium chloride), some are somewhat soluble (calcium
sulfate), some are somewhat insoluble (calcium carbonate), and some are very
insoluble (silver iodide).

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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