Precipitation Reactions: produce an insoluble solid product called a precipitate when two or more aqueous solutions are combined. It is easy to spot these at precipitates will almost form immediately and turn an otherwise clear (transparent) solution in an opaque one. Leave the solution long enough, and gravity will eventually settle the precipitate to the bottom and separate it from the solution altogether.
For example if an aqueous solution of cobalt (II) nitrate is mixed with sodium hydroxide, the two aqueous (soluble) solutions will react to form new compounds via a double displacement reaction, forming an insoluble product—the precipitate.
http://commons.wikimedia.org/wiki/File:Cobalt(II)hydroxide_precipitate.png c/o Capaccio
How do we predict products and the precipitate?
1. Write the products as they would form from a double displacement or exchange reaction. The two cations or anions switch places.
2. Identify the precipitate by using the solubility rules.
3. Assign other physical states: aqueous (aq) for all soluble substances; gas (g) for all gases formed; liquid (l) for all liquids. You can use the solubility table for predicting (aq) and (s).
4. Balance the chemical equation: