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 salt called a precipitate. The precipitate may stay in the solution as a suspension, fall out of solution on its own, or can be separated from the liquid using centrifugation, decantation, or filtration. The liquid that remains when a precipitate forms is called the supernate.
Whether or not a precipitation reaction will occur when two solutions are mixed may be predicted by consulting a solubility table or the solubility rules. Alkali metal salts and those containing ammonium cations are soluble. Acetates, perchlorates, and nitrates are soluble. Chlorides, bromides, and iodides are soluble. Most other salts are insoluble, with exceptions (e.g., calcium, strontium, barium sulfides, sulfates, and hydroxides are soluble).
Note that not all ionic compounds react to form precipitates. Also, a precipitate may form under certain conditions, but not others. For example, changes in temperature and pH can affect whether or not a precipitation reaction will occur. Generally, increasing temperature of a solution increases the solubility of the ionic compounds, improving the likelihood of precipitate formation. The concentration of the reactants is also an important factor.
Precipitation reactions are usually single replacement reactions or double replacement reactions. In a double replacement reaction, both ionic reactants dissociate in water and their ions bonds with the respective cation or anion from the other reactant (switch partners). In order for a double replacement reaction to be a precipitation reaction, one of the resulting products must be insoluble in aqueous solution. In a single replacement reaction, an ionic compound dissociates and either its cation or anion bonds with another ion in solution to form an insoluble product.
Uses of Precipitation Reactions
Whether or not mixing two solutions produces a precipitate is a useful indicator of the identity of the ions in an unknown solution. Precipitation reactions are also useful when preparing and isolating a compound.
Precipitation Reaction Examples
The reaction between silver nitrate and potassium chloride is a precipitation reaction because solid silver chloride is formed as a product.
AgNO3(aq) + KCl(aq) → AgCl(s) + KNO3(aq)
The reaction may be recognized as a precipitation because two ionic aqueous solutions (aq) react to yield a solid product (s).
It's common to write precipitation reactions in terms of the ions in the solution. This is called a complete ionic equation:
Ag+ (aq) + NO3−(aq) + K+ (aq) + Cl−(aq) → AgCl (s) + K+ (aq) + NO3−(aq)
Another way to write a precipitation reaction is as a net ionic equation. In the net ionic equation, the ions that don't participate in the precipitation are omitted. These ions are called spectator ions because they seem to sit back and watch the reaction without taking part in it. In this example, the net ionic equation is:
Ag+(aq) + Cl−(aq) → AgCl (s)
Properties of Precipitates
Precipitates are crystalline ionic solids. Depending on the species involved in the reaction, they may be colorless or colorful. Colored precipitates most often appear if they involve transition metals, including the rare earth elements.