Precipitation Reactions

Ionic solids may be water-soluble or water-insoluble. Water-soluble ionic solids dissolve by breaking up — or dissociating — into ions. Water molecules then surround the dissociated ions to form an aqueous solution. Water-insoluble ionic solids stay undissolved or undissociated in aqueous solutions.

If mixing aqueous solutions of two different ionic compounds results in a chemical reaction that produces an insoluble ionic solid — called the precipitate — it is a precipitation reaction. The precipitate stays undissolved and can be separated from the solution by filtration.

Sodium chloride and silver nitrate are water-soluble ionic compounds. An aqueous solution of sodium chloride contains sodium ions and chloride ions in their dissociated forms. Similarly, an aqueous solution of silver nitrate contains dissociated silver ions and nitrate ions.

When the two solutions are mixed, the resulting solution contains all four ions: the sodium and silver cations as well as the chloride and nitrate anions.

Since oppositely charged ions are attracted to each other, the cation from each reactant can pair with the anion from the other reactant to form new ionic products. Thus, sodium cations pair with nitrate anions to form sodium nitrate, and silver cations combine with chloride anions to form silver chloride.

Referring to the solubility rules allows us to determine whether the ionic products are water-soluble or water-insoluble. According to this guide, all nitrate salts are water-soluble without exception. Most chloride salts are water-soluble, but silver salt is an exception.

Therefore, sodium nitrate is a soluble salt that remains in the solution as dissolved sodium ions and nitrate ions, while silver chloride is an insoluble salt that precipitates from the solution.

The molecular equation for this precipitation reaction suggests that aqueous sodium chloride reacts with aqueous silver nitrate to form aqueous sodium nitrate and solid silver chloride.

This is an example of a salt metathesis reaction, which is also called a double displacement reaction. Salt metathesis is a chemical process in which the positive and negative ions of two ionic reactants exchange partners to form two new ionic products.

In this reaction, sodium and silver cations exchange partners — pairing with nitrate and chloride anions, respectively — to form sodium nitrate and silver chloride.