Precipitation (chemistry)

From Free net encyclopedia

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

Precipitation is the formation of a solid in a solution during a chemical reaction. This can occur when an insoluble substance is formed in the solution due to a reaction or when the solution has been supersaturated by a compound. In most situations, the solid forms ("falls") out of the solute phase, and sinks to the bottom of the solution (though it will float if it is less dense than the solvent, or form a suspension).

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. (Zumdahl, 2005)

    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^-

[edit]

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.

[edit]