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

Question

Owen Ambrose · Accepted Answer

This is function of solubility.

Naturally, we are speaking in generalities when we say that most halides, most alkali metal salts, most sulfates, most nitrates, most carbonates, and most phosphates are soluble in aqueous solution. The fact that the majority of carbonates and phosphates are insoluble is presumably due to the electrostatic interaction between the cation and anion; an anion with a charge larger than one unit should form stronger ionic bonds with the counterion, as evidenced by the solubilities; this also holds true for the general insolubilities of oxides and sulfides; naturally, hydrolysis is caused by the soluble oxides, which are those of the alkali metals. 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.

Noah Aldrich · Answer

undefined Because some combinations are only partially soluble, combining ions in solution can create insoluble compounds that cause a solid to precipitate.