A precipitation reaction, also known as a precipitate-forming reaction, is a type of double displacement reaction in chemistry where two aqueous solutions are mixed, and a solid substance (precipitate) forms as a result of the chemical reaction. Precipitation reactions typically involve the exchange of ions between the reactants, resulting in the formation of an insoluble compound that “falls out” of the solution as a solid.

The general chemical equation for a precipitation reaction is:

AB(aq) + CD(aq) → AD(s) + CB(aq)

Here’s what each component of the equation represents:

AB and CD are the reactants, typically aqueous solutions of ionic compounds.
AD is the solid precipitate formed as a result of the reaction.
CB represents any remaining ions that remain in solution, also typically in aqueous form.
Here are a couple of examples of precipitation reactions:

Precipitation of Silver Chloride (AgCl):

Reaction: AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
In this reaction, silver nitrate (AgNO3) and sodium chloride (NaCl) solutions are mixed. The result is the formation of silver chloride (AgCl) as a solid precipitate, which appears as a white, cloudy substance in the solution. Sodium nitrate (NaNO3) remains in the solution.
Precipitation of Barium Sulfate (BaSO4):

Reaction: Ba(NO3)2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaNO3(aq)
When barium nitrate (Ba(NO3)2) and sodium sulfate (Na2SO4) solutions are combined, barium sulfate (BaSO4) is formed as a solid precipitate. It appears as a white, finely divided solid. Sodium nitrate (NaNO3) remains dissolved in the solution.
Precipitation reactions are commonly used in analytical chemistry and laboratory procedures to separate and identify specific ions in a mixture. By observing the formation of a solid precipitate, chemists can detect the presence of certain ions in a solution and carry out further chemical analyses.