Precipitation (chemistry)

"Precipitate" redirects here, for the Interpol EP see Precipitate (EP).

Precipitation is the condensation of a solid from a solution during a chemical reaction. This occurs when the solution is supersaturated (this includes the obvious case that an insoluble compound is present). In this situation, the solid forms from the solute phase, and usually sinks to the bottom of the solution (though it will float if less dense than the solvent).

This effect is useful in many industrial and scientific applications whereby a chemical reaction may produce a solid that can be collected from the solution by various methods (e.g. filtration, decanting, centrifuging). Precipitation from a solid solution is also a useful way to strengthen alloys.

An important stage of the precipitation process is the onset of nucleation. The creation of a hypothetical solid particle includes the formation of an interface, which requires some energy based on the relative surface energy of the solid and the solution. If this energy is not available, and no suitable nucleation surface is available, supersaturation occurs.

An example of a precipitation reaction: Aqueous silver nitrate (AgNO3) is added to a solution containing potassium chloride (KCl) and the precipitation of a white solid, silver chloride is observed.

    AgNO3(aqueous) + KCl(aqueous) ---> AgCl(solid) + KNO3(aqueous)

The silver chloride (AgCl) has formed a solid, which is observed as a precipitate.

This reaction can be written emphasizing the dissociated ions in a combined solution

    Ag+, NO3-, K+, Cl- ---> AgCl(solid), K+, NO3-

Cation sensitivity

Precipitate formation is useful in the detection of the type of cation in an unknown salt. To do this, an alkali first reacts with the unknown salt to produce a precipitate which is the hydroxide of the unknown salt.

To identify the cation, the colour of the precipitate and its solubility in excess are noted.

Similar processes are often used to separate chemically similar elements, such as the rare earth metals.

Reference

Zumdahl, Steven S. <und>Chemical Principals.</und> 4th ed. New York: Houghton Mifflin Company, 2005.

External links

• http://www.hci.edu.sg/~limth/lessons/2002/qa/cations.htm