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

1 Answer
Jan 15, 2016

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


Typically, chemists exploit differential solubilities when using precipitation reactions.

All salts of the halides are soluble, except for those of #Ag^+#, #Pb^(2+)#, and mercurous ion, #Hg_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:

#NaCl(aq) + Ag(NO_3)(aq) rarr AgCl(s)darr + NaNO_3(aq)#

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:

#Ag^+ + Cl^(-) rarr AgCl(s)darr#

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.