Formation of a Precipitate

A precipitate forms in a double-replacement reaction when the cations from one of the reactants combine with the anions from the other reactant to form an insoluble ionic compound. When aqueous solutions of potassium iodide and lead (II) nitrate are mixed, the following reaction occurs.

There are very strong attractive forces that occur between Pb2+ and I− ions and the result is a brilliant yellow precipitate (Figure 1 ). The other product of the reaction, potassium nitrate, remains soluble.

Figure 1 : Lead (II) iodide precipitates when potassium iodide is mixed with lead (II) nitrate.