A precipitation reaction is a type of chemical reaction in which two or more substances in a solution react, forming an insoluble solid product that separates from the solution. The solid product is called a precipitate. Precipitation reactions are often used to separate and purify substances, as well as to identify different ions in a solution.

Here is an example of a precipitation reaction:

$$Pb(NO_3)_2_{(aq)} + 2KI_{(aq)} → PbI_2_{(s)} + 2KNO_3_{(aq)}$$

In this reaction, lead(II) nitrate and potassium iodide react to form lead(II) iodide, which is a solid that precipitates out of the solution. The other product of the reaction, potassium nitrate, remains dissolved in the solution.