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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.

## Example

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.

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