Precipitation (chemistry)
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Precipitation is the condensation of a solid from a solution during a chemical reaction. This occurs when the solution is supersaturated, whereupon the solid forms from the solute phase, and usually sinks to the bottom of the solution.
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.
Cation sensitivity
Precipitate formation is useful in detecting 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.