An acid is classified as "strong" when the concentration of its undissociated species is too low to be measured. Any aqueous acid with a pK value of less than 0 is almost completely deprotonated and is considered a strong acid. All such acids transfer their protons to water and form the solvent cation species (HO in aqueous solution) so that they all have essentially the same acidity, a phenomenon known as solvent leveling. They are said to be fully dissociated in aqueous solution because the amount of undissociated acid, in equilibrium with the dissociation products, is below the detection limit. = −1.4) behaves as a strong acid. When the ratio of water molecules to acid molecules in a solution of an acid is too small to fully solvate the dissociation products, dissociation is not complete, for example, with extremely concentrated solutions of sulfuric acid and hydrochloric acid.}}.
 --> Likewise, any aqueous base with an association constant pK less than about 0, corresponding to pK greater than about 14, is leveled to OH and is considered a strong base.
Nitric acid, with a pK value of around −1.7, behaves as a strong acid in aqueous solutions with a pH greater than 1. At lower pH values it behaves as a weak acid.
pK values for strong acids have been estimated by theoretical means. For example, the pK value of aqueous HCl has been estimated as −9.3.