A precipitate is a solid product which separates out from the solution during a chemical reaction. It can also be formed by passing a gas into an aqueous solution of a substance (like passing carbon dioxide into lime water).

Examples:

(i) Mixing of two clear solutions: Silver nitrate (AgNO₃) and sodium chloride (NaCl). A precipitate forms because the solid (AgCl) is insoluble in water. This is true for all precipitates; the solids are insoluble in aqueous solutions.

(ii) A yellow precipitate of lead iodide is formed at once, when potassium iodide solution is added to a solution of lead nitrate (ref fig).

Precipitation reactions occur all around us. Sometimes the pipes in our homes get clogged because precipitates of magnesium and calcium oxides have deposited themselves with in the pipes. This can happen with 'hard' water. Another example is the kidney stone. A kidney stone is nothing more than a precipitate – often of calcium ions (from cheese) and oxalates. It is often suggested that a good way to avoid kidney stones is to drink a lot of water. This helps because the solubility of the precipitate increases with the amount of water, thus avoiding the formation of kidney stone to begin with.