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Name:   Bito L.
Status:  student
Age:  13
Location: N/A
Country: N/A
Date: 2000


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?


Replies:
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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