Acid or base dissociation may better be described as ionization. Arrhenius theorized that an acid such as HCl would dissociate into H⁺ ions and Cl^- ions. Bronsted-Lowry theory has HCl reacting with the solvent to form H₃O⁺ ions and Cl^- ions. The degree of ionization determines the strength of an acid or base. Strong acids and strong bases are completely ionized. Determining the acidity (pH) of a solution of a strong acid or strong base is fairly straightforward.

There are 6 common strong acids in water: H₂SO₄, HNO₃, HClO₄, HCl, HBr, and HI. This list should be memorized.