= Precipitation
{disambiguate=chemistry}
{wiki}
<Precipitation (chemistry)> is formation of a <solid> from a <chemical solution>; the formed <solid> is a <precipitate>. Dissolved <chemical species> enter a separate <thermodynamic phase>. A newly formed <solid> can create a surface for <heterogeneous catalysis>; observing it does not establish the composition or catalytic role of that surface.
For example, mixing suitable <chemical solutions> of <calcium chloride> and <sodium carbonate> can form <calcium carbonate>:
$$
\mathrm{Ca}^{2+}(aq)+\mathrm{CO}_3^{2-}(aq)\longrightarrow\mathrm{CaCO}_3(s).
$$
Here $aq$ means dissolved in <water>, and $s$ marks the <solid>. Both sides contain one <calcium> <chemical atom>, one <carbon> <chemical atom>, and three <oxygen> <chemical atoms>; net <electric charge> is $+2-2=0$ on the left and zero on the right. The arrow describes the net change when <precipitation (chemistry)> occurs, rather than promising complete removal of either <ion>. The <calcium carbonate precipitation experiment> makes this phase separation visible.<Carbon Captured Chalk by Tony Butterfield>[Carbon Captured Chalk, Summary and Background for Teachers: sodium carbonate and calcium chloride form calcium carbonate]{ref}
The attraction is that an invisible dissolved inventory becomes a recoverable <solid>. <Net ionic equations> track that transfer; the <saturation criterion for precipitation> determines whether it is favored under specified conditions. This combination supports separations and chemical analysis.<Chemistry 2e by Paul Flowers and others>[Chemistry 2e, §4.2, Precipitation Reactions and Solubility Rules: separations and analytical uses]{ref}
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