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Name: Linda S.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 10/16/2004


Question:
Why are some salts soluble and others insoluble. Example is AgCl and NaCl. This is a grade 10 level question.


Replies:
If I had a really good answer to this at any level, I would be very ealthy!!

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 complicated.

Vince Calder



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