The foregoing is the most useful everyday definition of acids and bases, applicable to aqueous solution. The Bronsted-Lowry theory goes one step further, and frees us from a dependence on water as a solvent. It also is helpful in explaining the difference between strong and weak acids.

According to Bronsted and Lowry, an acid is any substance that releases protons in solution, and a base is any substance that combines with protons and removes them from solution.

Thus in the dissociation of HCl, HCl = H⁺ + Cl^-, the molecule of HCl is a B-L (Bronsted-Lowry) acid because it can release a proton, and the Cl^- ion is a B-L base because it can combine with a proton. HCl and Cl^- are called a conjugate acid-base pair (see diagram on previous page). In the two-proton dissociation of sulfuric acid,

the bisulfate ion, HSO₄^-, is the conjugate base of the B-L acid H₂SO₄, and at the same time is the conjugate acid of the B-L base SO^2-. The words "acid" and "base" in the Bronsted-Lowry theory do not describe what a molecule is, but rather what it does.