Precipitate Definition and Example in Chemistry

Chemistry Glossary Definition of Precipitate

Diagram illustrating the process of chemical precipitation
This diagram illustrates the process of chemical precipitation. ZabMilenko/Wikipedia/Public Domain

In chemistry, to precipitate is to form an insoluble compound either by reacting two salts or by changing the temperature to affect the solubility of the compound. Also,​ the name given to the solid that is formed as a result of a precipitation reaction.

Precipitation may indicate a chemical reaction has occurred, but it may also occur if solute concentration exceeds its solubility. Precipitation is preceded by an event called nucleation, which is when small insoluble particles aggregate with each other or else form an interface with a surface, such as the wall of a container or a seed crystal.

Precipitate vs Precipitant

The terminology can seem a bit confusing. Here's how it works: Forming a solid from a solution is called precipitation. A chemical that causes a solid to form in a liquid solution is called a precipitant. The solid is called the precipitate. If the particle size of the insoluble compound is very small or there is insufficient gravity to draw the solid to the bottom of the container, the precipitate may be evenly distributed throughout the liquid, forming a suspension. Sedimentation refers to any procedure that separates the precipitate from the liquid portion of the solution, which is called the supernate. A common sedimentation technique is centrifugation. Once the precipitate has been recovered, the resulting powder may ​be called a "flower".

Precipitation Example

Mixing silver nitrate and sodium chloride in water will cause silver chloride to precipitate out of solution as a solid. In this example, the precipitate is silver chloride.

When writing a chemical reaction, the presence of a precipitate may be indicated by following the chemical formula with a down arrow:

Ag+ + Cl- → AgCl↓

Uses of Precipitates

Precipitates may be used to identify the cation or anion in a salt as part of qualitative analysis. Transition metals, in particular, are known to form different colors of precipitates depending on their elemental identity and oxidation state. Precipitation reactions are used to remove salts from water, to isolate products, and to prepare pigments.

Precipitate Aging

A process called precipitate aging or digestion occurs when a fresh precipitate is allowed to remain in its solution. Typically the temperature of the solution is increased. Digestion can produce larger particles with a higher purity. The process that leads to this result is known as Ostwald ripening.