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<div>: ''"Precipitate" redirects here, for the [[Interpol (band)|Interpol]] [[Extended play|EP]] see [[Precipitate (EP)]].''<br />
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'''Precipitation''' is the formation of a [[solid]] in a [[solution]] during a [[chemical reaction]]. When the chemical reaction occurs the solid formed is called the '''precipitate'''. This can occur when an insoluble substance, the '''precipitate''', is formed in the solution due to a reaction or when the solution has been [[supersaturated]] by a compound. The formation of a precipitate is a sign of a chemical change. In most situations, the solid forms ("falls") out of the solute phase, and sinks toPENIS PENIS PENIS PENIS PENIS PENIS the bottom of the solution (though it will float if it is less dense than the solvent, or form a [[suspension (chemistry)|suspension]]).<br />
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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 [[precipitation strengthening|strengthen]] [[alloy]]s.<br />
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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.<br />
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An example of a precipitation reaction: Aqueous silver nitrate (AgNO<sub>3</sub>) is added to a solution containing potassium chloride (KCl) and the precipitation of a white solid, silver chloride is observed. (Zumdahl, 2005)<br />
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:AgNO<sub>3</sub>(aq) + KCl(aq) → AgCl(s) + KNO<sub>3</sub>(aq)<br />
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The silver chloride(AgCl) has formed a solid, which is observed as a precipitate.<br />
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This reaction can be written emphasizing the dissociated ions in a combined solution<br />
:Ag<sup>+</sup>(aq) + NO<sub>3</sub><sup>-</sup>(aq) + K<sup>+</sup>(aq) + Cl<sup>-</sup>(aq) → AgCl(solid) + K<sup>+</sup>(aq) + NO<sub>3</sub><sup>-</sup>(aq)<br />
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A final way to represent a precipitate reaction is known as a ''net ionic reaction''. In this case, any spectator ions (those which do not contribute to the reaction) are left out of the formula completely. This simplifies the above equations to the following:<br />
:Ag<sup>+</sup>(aq) + Cl<sup>-</sup>(aq) → AgCl(s)<br />
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==Cation sensitivity==<br />
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. <br />
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To identify the cation, the colour of the precipitate and its solubility in excess are noted.<br />
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Similar processes are often used to separate chemically similar elements, such as the [[rare earth]] metals.<br />
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==Reference==<br />
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Zumdahl, Steven S. ''Chemical Principles.'' 4<sup>th</sup> ed. New York: Houghton Mifflin Company, 2005.<br />
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==External links==<br />
* http://www.hci.edu.sg/~limth/lessons/2002/qa/cations.htm<br />
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[[Category:Chemical processes]]<br />
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