Aqueous silver nitrate (AgNO3) is added to a solution containing potassium chloride (KCl), and the precipitation of a white solid, silver chloride (AgCl), is observed:
AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3(aq)
Note that the product silver chloride is the precipitate, and it is designated as a solid. This reaction can be also be written in terms of the individual dissociated ions in the combined solution. This is known as the complete ionic equation:
Ag+ (aq) + NO3−(aq) + K+ (aq) + Cl−(aq) → AgCl (s) + K+ (aq) + NO3−(aq)
A final way to represent a precipitation reaction is known as the net ionic equation. In this case, any spectator ions (those that do not contribute to the precipitation reaction) are left out of the formula completely. Without the spectator ions, the reaction equation simplifies to the following:
Ag+(aq) + Cl−(aq) → AgCl (s)
Observing precipitation reactions can be useful in the laboratory to determine the presence of various ions in solution. For instance, if silver nitrate is added to a solution of an unknown salt and a precipitate is observed, the unknown solution might contain chloride (Cl–).
