Some reactions are driven by the formation of a solid precipitate from a solution. In order to determine which substances will precipitate we need to be familiar with the solubility trends for common ionic substances. Substances that are soluble in water will not form precipitates, whereas substances that are insoluble will form precipitates.

Solubility Rules

The top portion of Table 1 describes substances that are soluble, along with a few exceptions. Ionic compounds in which the cation is NH_4^+ , Li^+ , Na^+ , K^+ , Rb^+ , or Cs^+ will always be soluble, with no exceptions (i.e., the identity of the anion doesn’t matter). Similarly, ionic compounds in which the anion is nitrate ( NO_3^- ), perchlorate ( ClO_4^- ), or acetate ( CH_3COO^- ) will always be soluble, with no exceptions (i.e., the identity of the cation doesn’t matter). These soluble anions and cations share the common property that they are relatively large and singly charged and, as you would expect from Coulomb’s law, only form weak interactions with any counter ion. Ionic compounds in which the anion is chloride ( Cl^- ), bromide ( Br^- ), or iodide ( I^- ) are soluble unless the cation is mercury(I) ( Hg_2^{2+} ), silver ( Ag^+ ), or lead ( Pb^{2+} ). For example, aluminum chloride ( AlCl_3 ) would be soluble, but silver chloride (AgCl) would not be soluble. Ionic compounds in which the anion is sulfate ( SO_4^{2-} ) are usually soluble unless the cation is Ca^{2+}, Sr^{2+}, Ba^{2+}, Hg_2^{2+}, Ag^+, \text{or } Pb^{2+} .

The bottom part of Table 1 describes substances that are insoluble, along with a few exceptions. Ionic compounds in which the anion is carbonate ( CO_3^{2-} ) or phosphate ( PO_4^{3-} ) are insoluble unless the cation is one of those we listed earlier whose salts are always soluble ( NH_4^+, Li^+, Na^+, K^+, Rb^+, \text{or }Cs^+ ). Ionic compounds in which the anion is hydroxide ( OH^- ) or sulfide ( S^{2-} ) are also insoluble unless the cation is NH_4^+, Li^+, Na^+, K^+, Rb^+, Cs^+, Ca^{2+}, Sr^{2+}, \text{or }Ba^{2+} (the “always soluble” cations we listed earlier, plus a couple more).

Table 1: Solubility Rules

If two solutions are mixed together that contain an insoluble cation/anion pair, then the pair of ions will react and form an ionic solid that will precipitate from the solution. For example, If a solution of ammonium sulfate is mixed with a solution of barium chloride, the barium ions and sulfate ions will react to form solid barium sulfate. Ammonium chloride, however, is soluble, so the ammonium and chloride ions would not form a precipitate and would instead remained dissolved in the solution as spectator ions.

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