Compounds that, upon ionization, yield cations consisting entirely of hydrogen ions (H⁺) are termed acids; alternatively, substances that dissolve in water and are capable of releasing protons to form H₃O⁺ (hydronium ions) are also classified as acids. The higher the concentration of H₃O⁺, the stronger the acidity of the solution. H₃O⁺ ions are present even in pure water, where their concentration stands at 10⁻⁷ mol/L. This phenomenon results from the transfer of protons from one water molecule to another. In the traditional sense, the concentration of H₃O⁺ is also considered dependent upon the concentration of hydrogen ions, although the vast majority of hydrogen ions in aqueous solutions exist in the form of H₃O⁺.
In chemistry, the narrow definition of an acid is: a compound that, when dissolved in an aqueous solution, undergoes ionization to yield cations consisting exclusively of hydrogen ions. Proposed by Arrhenius, this theoretical framework is known as the Arrhenius acid-base theory.
