Precipitate: definition in chemistry
Precipitate: An insoluble solid that forms from a solution during a chemical reaction. As a verb, to precipitate means to come out of solution as a solid.
A precipitate forms when two solutions are mixed and some of their ions combine to make a compound that does not dissolve in water. It may appear as cloudiness, a colored solid or a layer that settles to the bottom. Chemists use precipitation reactions to identify ions, remove pollutants from water and make pigments, and solubility rules help predict when a precipitate will form.
Examples
- Silver nitrate + sodium chloride gives white silver chloride (AgCl)
- Lead(II) nitrate + potassium iodide gives bright yellow lead iodide (PbI₂)
- Soap scum forms when soap reacts with calcium ions in hard water
- Limewater turns cloudy with CO₂ as calcium carbonate (CaCO₃) forms
On the periodic table
Solubility depends on the ions involved: compounds of group 1 metals are almost always soluble, so they rarely form precipitates.