Question

Why do some mixtures of ionic compounds form a solid precipitate?

Answer 1

This is function of solubility.

Explanation:

In aqueous solution most halides are soluble; most alkali metal salts are soluble; most sulfates are soluble; most nitrates are soluble; most carbonates are insoluble; most phosphates are insoluble. Of course, we speak of generalities.

That most phosphates and most carbonates are insoluble is undoubtedly a consequence of the electrostatic interaction between anion and cation. An anion with greater than unit charge should form stronger ionic bonds with the counterion; and this is manifested in the solubilities. The same reasoning applies to the general insolubilities of oxides and sulfides. Of course, the soluble oxides, those of the alkali metals, cause hydrolysis.

Now solubility is an experimental phenomenon, and there are exceptions to the rules I have given. For instance while most halides are soluble, `PbX_2`, `AgX`, and `Hg_2X_2` are insoluble in aqueous solution. As an undergraduate, this is something to note and recall.