When two aqueous solutions are mixed and a solid, the precipitate, is formed, a precipitation reaction has occurred. However, not all solutions that are mixed together will form a precipitate. When calculating if a precipitate will occur in a given equation, if Q, the initial concentration, is greater than K, the equilibrium concentration, then a precipitate will form because there is excess reactants present in the mechanism. Whether or not a precipitate forms depends upon the solubility rules, so keep them in mind as you try to do these problems.
1) NaCl is an aqueous solution (see solubility rules), when mixed with Pb(NO3)2, it becomes:
2NaCl(aq) + Pb(NO3)2(aq) —> PbCl2 (s) + 2NaNO3(aq)
In this case, NaCl and Pb(NO3)2 create a precipitate which is PbCl. According to the solubility rules, Cl– is insoluble when it is with Pb2+.
2) A solution of Cu(NO3)2(aq) is mixed with a solution of (NH4)2SO4(aq). Does it form a precipitate?
The equation for this reaction is:
Cu(NO3)2(aq) + (NH4)2SO4(aq) —> NH4NO3(aq) + CuSO4(aq)
In this case, because nitrates are always soluble, and NH4+is always soluble, no precipitate forms.