Precipitation Reactions

A precipitation reaction is a reaction that yields an insoluble product—a precipitate—when two solutions are mixed. When a colorless solution of silver nitrate is mixed with a yellow-orange solution of potassium dichromate a reddish precipitate of silver dichromate is produced.

This unbalanced equation has the general form of an exchange reaction:

Thus precipitation reactions are a subclass of exchange reactions that occur between ionic compounds when one of the products is insoluble. Because both components of each compound change partners, such reactions are sometimes called double-displacement reactions. Two important uses of precipitation reactions are to isolate metals that have been extracted from their ores and to recover precious metals for recycling.

Just as important as predicting the product of a reaction is knowing when a chemical reaction will not occur. Simply mixing solutions of two different chemical substances does not guarantee that a reaction will take place. For example, if 500 mL of aqueous NaCl solution is mixed with 500 mL of aqueous KBr solution, the final solution has a volume of 1.00 L and contains Na+(aq)., Cl−(aq)., K+(aq)., and Br−(aq). As you will see in (Figure 1

), none of these species reacts with any of the others. When these solutions are mixed, the only effect is to dilute each solution with the other.

Figure 1 : The Effect of Mixing Aqueous KBr and NaCl Solutions. Because no net reaction occurs, the only effect is to dilute each solution with the other. (Water molecules are omitted from molecular views of the solutions for clarity.)