Factors for Salt Solubility
Name: Linda S.
Why are some salts soluble and others insoluble. Example
is AgCl and NaCl. This is a grade 10 level question.
If I had a really good answer to this at any level, I would be very
But the general principles are these: To dissolve a substance (any
substance, including salts) I have to break the interatomic/intermolecular
forces holding the substance together. This costs energy. I get back energy
if the interaction between the solute and solvent is sufficiently strong.
And if I get back more energy than it 'costs' to break the forces holding
the solute together, it dissolves. Now there are two problems with this
"explanation". First, the clever student is going to object that solutes
should all be either soluble completely, or insoluble completely. And that
objection is true. But that is the "fault" of the explanation, which is
incomplete. Second, there is another driving force involved -- the magic
term 'entropy' increase. Gases are miscible (completely soluble) even
though there is no energy gain or loss. The same 'driving force' operates
with other solutes, but is much more subtle. There are any number of
examples (ammonium chloride) that absorb energy when dissolved, so the
energy "explanation" is clearly incomplete. Models and theories of
solubility of even "simple" systems are rather incomplete and often very
complicated mathematically. If anyone has a good explanation that is
credible, I would like to know it too.
An easier related problem, but still a difficult one, is the volatility
of various substances. Here there is no "getting back energy" from the
interaction of the solute and solvent, just "pulling the molecules" out of
the liquid and evaporating them into a gas. Even this problem is very
Click here to return to the Chemistry Archives
Update: June 2012