An ion is an atom or molecule that carries an electrical charge. Cations are positively-charged ions created by the loss of electrons. Anions are negatively-charged ions created by the gain of electrons. In chemical reactions, all ions exhibit their own unique, characteristic behaviors.
To identify a particular ion in a solution, the solution must be subjected to a series of chemical tests. The results of the tests are compared to the results of the same test performed on a solution containing a known ion. This process of testing is called qualitative analysis. The reagents used in analysis tests vary widely, but the most common are 6M solutions of hydrochloric acid (HCL), nitric acid (HNO3), sodium hydroxide (NaOH) and ammonia (NH3).
When an ionic solution is mixed with a reagent, the ions within the mixture may combine, forming a new compound or new ions. These new compounds or ions may cause physical, easily observed changes in the solution. Common results of qualitative analysis tests include the release of a gas, the formation of a precipitate or a change in the color of the solution.
Precipitation occurs when two solutions are mixed together and the ions in the solution combine to form solid, insoluble particles of a new compound. For example, to test for the presence of the silver cation (Ag+) in the solution, add hydrochloric acid to the solution and check for the formation of a precipitate. Treating a solution of silver nitrate with hydrochloric acid will produce a white precipitate as the silver cations (Ag+) combine with the chloride anions (Cl-) to form the insoluble solid, silver chloride (AgCl).
Many ions in aqueous solutions have a characteristic color. For example, a solution containing chromium (Cr3+) appears blue-green. A solution containing the cation of iron Fe2+ is light blue, while Fe3+ is yellow-brown. Mixing colorless ferric chloride (FeCl3) solution with potassium thiocyanate (KSCN) causes the formation of the thiocyanatoiron ion ([FeSCN]2+), which turns the color of the solution to a dark, blood red.
Verifying the release of gas from a chemical reaction may involve more than one type of observation. For example, mixing sodium hydroxide (NaOH) with ammonium chloride (NH4C1) will create ammonia gas (NH3(g)). This gas may be seen as bubbles in the solution. A strong odor of ammonia will be detected as the gas escapes the test tube. And finally, a strip of moistened red litmus paper held over the test tube will turn blue in the presence of the ammonia.