A precipitation reaction is a type of double displacement reaction in which two soluble ionic substances, normally salts, are combined. Ionic compounds are those which, in solution, separate to form charged ions. These soluble compounds react to form another soluble compound and an insoluble compound. This insoluble compound is called a precipitate.
An acid-base neutralization reaction describes a double displacement reaction in which an acid and a base chemically react with one another. When there is an equal amount of acid and base, these two compounds neutralize each other, forming water and a salt. This occurs because an acidic compound forms hydronium ions in solution whereas a basic compound forms hydroxide ions. These hydronium and hydroxide ions combine to form water, thus neutralizing the pH of the solution.
A common precipitation reaction involves the formation of barium sulfate from the combination of an aqueous solution of barium chloride and sodium sulfate. In this reaction the chloride ions are displaced by sulfate ions, forming barium sulfate, leaving the chloride ions to combine with the sodium ions, forming sodium chloride. The precipitant, therefore, is the solid barium sulfate formed from the displacement of two of the ions in the reactant molecules: BaCl2 (aq) + Na2SO4 (aq) -> BaSO4 (s) + 2NaCl (aq).
The combination of hydrochloric acid and sodium hydroxide is a common example of a neutralization reaction. In aqueous solution the hydronium ions from the hydrochloric acid and the hyrdroxide ions from the sodium hydroxide combine, forming water. The remaining sodium and chloride ions that are displaced in the reaction then combine and form a salt:
HCl + NaOH -> NaCl + H2O.