What Is a Precipitation Reaction?
A precipitation reaction is a type of chemical reaction in which two soluble salts in aqueous solution combine and one of the products is an insoluble solid called a precipitate.
These reactions typically occur during double displacement (or metathesis) reactions:
AB(aq) + CD(aq) → AD(s) + CB(aq)
How Does It Work?
When two ionic compounds dissolve in water, they dissociate into their respective ions. If the combination of cations and anions from the two solutions forms an insoluble compound, that compound will come out of solution as a solid precipitate.
Solubility rules help predict whether a precipitate will form. For example:
- Most nitrate (
NO₃⁻) salts are soluble. - Most chloride (
Cl⁻) salts are soluble, except those of silver, lead, and mercury. - Most sulfate (
SO₄²⁻) salts are soluble, except those of barium, calcium, and lead.
Common Example
A classic precipitation reaction occurs between silver nitrate and sodium chloride:
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Silver chloride (AgCl) is insoluble and appears as a white precipitate.
Applications
Precipitation reactions are used in:
- Qualitative analysis in laboratories
- Water treatment to remove impurities
- Industrial synthesis of pigments and materials