A type of chemical reaction in which two anions or cations switch places between two compounds. An example of such a reaction is the reaction between sodium carbonate and calcium chloride, both in aqueous solution:
Na2CO3(aq) + CaCl2(aq) ------> CaCO3(ppt) + 2NaCl(aq)
Most double displacement reactions are like the one above: two soluble compounds react to form one soluble and one insoluble compound. The insoluble compound then precipitates out of solution.
Acid-Base reactions are also double displacement. An example is the reaction between hydrochloric acid and sodium hydroxide:
NaOH(aq) + HCl(aq) -----> NaCl(aq) + H2O(l)
Even acid-base reactions are not exempt from the "precipiate rule". The liquid water produced is not in solution (it is the solvent), and therefore can be considered a precipitate. THERE ARE VERY FEW DOUBLE DISPLACEMENT REACTIONS IN WHICH NO PRECIPITATE IS PRODUCED.