# What is a precipitate, and how can they be used to separate solutes?

Jan 15, 2016

A precipitate is a solid or a solid phase that separates from solution.

#### Explanation:

Typically, chemists exploit differential solubilities when using precipitation reactions.

All salts of the halides are soluble, except for those of $A {g}^{+}$, $P {b}^{2 +}$, and mercurous ion, $H {g}_{2}^{2 +}$. Suppose you had a solution of (soluble!) silver nitrate and wished to isolate the silver ion. Addition of sodium chloride would result in a curdy white precipitate of silver chloride:

$N a C l \left(a q\right) + A g \left(N {O}_{3}\right) \left(a q\right) \rightarrow A g C l \left(s\right) \downarrow + N a N {O}_{3} \left(a q\right)$

Actually I can be even more concise than this because the sodium and nitrate ions are simply along for the ride, and do not undergo chemical change in solution. Thus I can write the net ionic equation:

$A {g}^{+} + C {l}^{-} \rightarrow A g C l \left(s\right) \downarrow$

Of course, you have to know which salts are soluble, and thus which mixtures will produce an insoluble salt. There should be many resources here and on the web. Good luck.