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If you dissolve two different ionic compounds (that don't make a precipitate) in water and then boil the solution so all the water evaporates, the salts will re-crystallize. Is it possible that you will get two compounds that are different from the ones you orignially dissolved? Is there any way to predict whether you will get the original compounds or new ones?

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Mixing and matching different anions and cations can easily produce different compounds. For instance, sodium and potassium chloride, and sodium and potassium nitrate, are all soluble in water and do not react to form a precipitate. When the aqueous solution dries, all four compounds are possible. So, mixing sodium chloride and potassium nitrate will result in 2 anions and 2 cations in solution, allowing all four pairs to possible precipitate.

Now, will the precipitate as pure compounds or not is another question. That requires looking at the relevant phase diagrams and seeing what is thermodynamically favored (and consideration of kinetic factors like how fast you evaporate the liquid).

As an example, the phase diagram for KCl and NaCl exhibits a miscibility gap starting at 505C. This means that below that temperature they will phase separate into a Na-rich chloride and a K-rich chloride. At 100C the concentration of the 'impurity' phase is less than 1%.

The nitrate diagram also shows a two phase region below 100C.

So, mixing (Na,K) (Cl,NO3) will result in all four precipitating out.

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