Evaporation of a mixture of salts I was wondering what happens when you boil dry a solution of several salts for example a solution of KCl and NaNO3. Since the ions dissociate I was wondering if the ions would combine randomly upon being boiled out of solution making the original salts as well as KNO3 and NaCl? Does this happen? If not why?

Also in a related question when you mix two solutions to form a precipitate in a double displacement reaction will the insoluble salt always form or are there situations where an insoluble salt can form but doesn't?

Thanks in advance.
As the solvent evaporates the least soluble salts precipitate in succession.  This is used during seawater and salt lake evaporations to isolate values beyond NaCl. Overall, you will get a mixture of combinations of cations and anions.

Supersaturation is not uncommon.  Metastable solutions usually require nucleation sites to get over their initial activation energy for solids' crystallization.  High melting points (strong lattice forces) are difficult to supersaturate.  Benzil in xylenes remarkably supersaturates, and is stable for a week or more in silanized glassware and cooled very slowly within styrofoam and rags.