→ this method is used to prepare insoluble salts. → involves mixing 2 solutions to form an insoluble solid that separates out from the reaction mixture. → also known as ionic precipitation
General equation: Aqueous solution AB + aqueous solution XY → insoluble salt AY + aqueous solution XB a) dilute acids: aqueous states b) soluble salts: aqueous states c) insoluble salts: solid state
Features of precipitation reactions
- involves 2 reactants (solutions)
- Reactants are ionic (made up of cations and anions)
- Reactants must be in aqueous solutions (hence reactants must be soluble in water) containing in the ions that will form the insoluble salt.
- This reaction produces 2 products. One of the products is a solid ionic compound which is insoluble in water. The insoluble product is known as the precipitate. The other product is made up of cations and anions (spectator ions) and is an aqueous solution
Important note: precipitate can only be used for solid substances that are produced by reactions where all reactants are aqueous solutions.
to precipitate an insoluble salt, AY, we can mix 2 aqueous solutions:
- AB(contains the A+ cation)
- XY (contains the Y- anion)
when the 2 solutions are mixed → insoluble salt AY and aqueous solution XB are formed. aqueous solution of AB + aqueous solution of XY → insoluble precipitate of AY + aqueous solution of XB → this method is used to separate the insoluble salts since slats can be separated from starting materials by filtration.
one reactant must have the cation of the insoluble salt, and the other reactant must have the anion of the insoluble salt.
you can use 2 soluble salts to form 1 insoluble salt***
Procedure
- add excess sodium sulfate solution to lead(II)nitrate solution and stir constantly until no more precipitate is formed
- filter to obtain the residue which is the precipitate of lead(II)sulfate
- dry the precipitate between a few sheets of filter paper
- wash the precipitate with a little cold distilled water to remove the impurities.
Generic Steps:
- Mix the solutions together in a beaker. (stir until no more precipitate is formed)
- Filter the mixture to obtain the precipitate/residue
- wash the precipitate/residue with cold distilled water
- dry the precipitate/residue between 2 pieces of filter paper.
The 2 reactants that can be used to prepare a salt by the precipitation method are:
- Nitrate; because all nitrates are soluble in water
- Salt containing the sodium ion or potassium ion, since all sodium and potassium salts are soluble. alternatively, acids could be used.