Do silver nitrate and potassium phosphate form a precipitate when mixed together?
When silver nitrate and potassium phosphate are combined, they form a double replacement reaction. In a double replacement reaction, the ions 'switch partners.' The balanced chemical reaction between silver nitrate and potassium phosphate is as follows:
3AgNO3(aq) + K3PO4(aq) -> Ag3PO4(s) + 3KNO3(aq)
The product silver phosphate is a precipitate as indicated by the (s) for solid. In order to determine if a reaction will form a precipitate or not, you need to refer to the solubility rules. From the solubility rules, you can find that silver phosphate will form a precipitate because there is a rule that states that all phosphates are insoluble, except for those with NH4 or with alkali metals. This explains why in the reactants, K3PO4 is soluble, because potassium is an alkali metal in group 1. Silver, however, is not an alkali metal, so the product silver phosphate will form a precipitate.
When determining if a precipitate will be formed, you will often be asked to write a net ionic equation which shows the ions that combine to form the precipitate. For this equation, the net ionic equation is:
3 Ag+(aq) + PO4 3–(aq) → Ag3PO4(s)
The reference link below contains a good explanation of all of the solubility rules you will need to solve other precipitate problems.