A strong acid is defined as an acid that undergoes complete ionization in an aqueous solution (with polyprotic acids, such as sulfuric acid, being notable exceptions). Alternatively, when understood through the concept of acidity constants, a strong acid is defined as one with a pKa value of less than 1.74. This value implies that, under standard conditions, the concentration of hydrogen ions is equivalent to the concentration of the acid added to the solution.
Most strong acids are corrosive; however, there are exceptions. For instance, carborane acid (H(CHB11Cl11))-a type of superacid-is a million times more acidic than sulfuric acid yet is completely non-corrosive. Conversely, hydrofluoric acid (HF)-classified as a weak acid-is highly corrosive. It is capable of dissolving the vast majority of metal oxides-including glass-as well as all metals with the exception of iridium.
The chemical equation representing the complete dissociation of a strong acid in an aqueous solution is as follows: HA(aq) + H2O(l) → H3O+(aq) + A⁻(aq)
Generally, acids do not dissociate completely in water; consequently, their reactions are typically represented as chemical equilibria rather than as complete reactions. Weak acids are, by definition, those that do not undergo complete dissociation. Using acidity constants to distinguish between strong and weak acids is not always straightforward (as the numerical differences can be difficult to interpret or appear subtle); therefore, employing chemical equations to differentiate between the two categories is often a more logical approach.
Since strong acids undergo complete dissociation in aqueous solutions, the concentration of hydrogen ions in the water is equivalent to the initial concentration of the acid introduced into the solution: [HA] = [H⁺] = [A⁻]; pH = -log[H⁺]
