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.
Key Takeaways: Precipitate Definition in Chemistry
- In chemistry, precipitate is both a verb and a noun.
- To precipitate is to form an insoluble compound, either by decreasing the solubility of a compound or by reacting two salt solutions.
- The solid that forms via a precipitation reaction is called the precipitate.
- Precipitation reactions serve important functions. They are used for purification, removing or recovering salts, for making pigments, and to identify substances in qualitative analysis.
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".
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. Under controlled conditions, a precipitation reaction produces pure crystals of precipitate. In metallurgy, precipitation is used to strengthen alloys.
How to Recover a Precipitate
There are several methods used to recover a precipitate:
Filtration: In filtration, the solution containing the precipitate is poured over a filter. Ideally, the precipitate remains on the filter, while the liquid passes through it. The container may be rinsed and poured onto the filter to aid recovery. There is always some loss, either through dissolution into the liquid, passing through the filter, or adhesion to the filter media.
Centrifugation: In centrifugation, the solution is rapidly rotated. For the technique to work, the solid precipitate must be more dense than the liquid. The compacted precipitate, called the pellet, may be obtained by pouring off the liquid. There is typically less loss than with filtration. Centrifugation works well with small sample sizes.
Decantation: In decantation, the liquid layer is poured or suctioned away from the precipitate. In some cases, an additional solvent is added to separate the solution from the precipitate. Decantation may be used with the entire solution or following centrifugation.
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.
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- Zumdahl, Steven S. (2005). Chemical Principles (5th ed.). New York: Houghton Mifflin. ISBN 0-618-37206-7.