Why is \(\mathrm{H}_{3} \mathrm{O}^{+}\) the strongest acid and \(\mathrm{OH}^{-}\) the strongest base that can exist in significant amounts in aqueous solutions?

An acid is a substance that donates a proton (H⁺ ion) when dissolved in water. A base is a substance that accepts a proton when dissolved in water. Acids and bases react with water in a process called ionization. During this process, water molecules break apart into ions, specifically H₃O⁺ (hydronium ion) and OH⁻ (hydroxide ion). In water, the following equilibrium reaction occurs: \[2 H_{2}O \rightleftharpoons H_{3}O^{+} + OH^{-}\]

The strength of an acid or a base is determined by its ability to donate or accept protons (H⁺ ions). Strong acids easily donate protons, while strong bases easily accept protons. The ionization process follows an equilibrium, meaning the reaction proceeds in both directions. The equilibrium constant (K) of the reaction is given by: \[K = \frac{[H_{3}O^{+}][OH^{-}]}{[H_{2}O]^{2}}\] In pure water, the equilibrium constant is called the ion product of water (Kw), which is approximately 1.0 x 10⁻¹⁴ at 25°C. This value remains constant for any aqueous solution.

The strength of an acid is closely related to the strength of the conjugate base it forms by losing a proton. Likewise, the strength of a base is related to the strength of the conjugate acid it forms by gaining a proton. In aqueous solutions, H₃O⁺ and OH⁻ are the strongest acid and base that can exist in appreciable amounts because they are the ions responsible for establishing the equilibrium constant (Kw). When H₃O⁺ forms from a stronger acid, it will donate a proton back to the conjugate base, reverting the stronger acid to its original form. Therefore, it is not possible for a stronger acid than H₃O⁺ to exist in significant amounts in aqueous solutions. Similarly, when OH⁻ forms from a stronger base, it will accept a proton from the conjugate acid, reverting the stronger base to its original form. Therefore, it is not possible for a stronger base than OH⁻ to exist in significant amounts in aqueous solutions.

In aqueous solutions, H₃O⁺ and OH⁻ are the strongest acid and base that can exist in significant amounts because they represent the fundamental ions responsible for establishing the equilibrium constant. Any stronger acid or base would quickly react with the conjugate base or acid, returning to its original form. This behavior prevents any stronger acids or bases from existing in significant amounts in aqueous solutions.