A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.

For example :

$\mathop {N{a_2}C{O_{3(aq)}}}\limits_{{\text{Sodium carbonate}}}  + \mathop {CaC{l_2}_{(aq)}}\limits_{{\text{Calcium chloride}}}  \to \mathop {CaC{O_3}_{\left( s \right)}}\limits_{{\text{Calcium}}\,{\text{carbonate}}}  + \mathop {2NaC{l_{(aq)}}}\limits_{{\text{Sodium}}\,{\text{chloride}}\,\,} $

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

Another example of precipitation reaction is :

$\mathop {N{a_2}S{O_{4(aq)}}}\limits_{{\text{Sodium sulphate}}} \, + \mathop {\,BaC{l_2}_{(aq)}\,}\limits_{{\text{Barium chloride}}}  \to \,\mathop {BaS{O_{4(s)}}}\limits_{{\text{Barium sulphate}}} \, + \,\mathop {2NaC{l_{(aq)}}\,}\limits_{{\text{Sodium chloride}}} $

In this reaction, barium sulphate is obtained as a precipitate.