水：ブレンステッド・ローリーの酸塩基理論

An amphoteric molecule is capable of acting as both an acid and a base. Water is an example of an amphoteric molecule. In an aqueous solution of ammonia, water can act as an acid by donating a proton to ammonia, a base. In contrast, water acts as a base in an aqueous solution of hydrochloric acid by accepting a proton from the acid. Because it is amphoteric, water can undergo autoionization. In this process, one molecule of water acts as the acid by donating a proton to another molecule of water that acts as a base by accepting that proton. This results in the production of a hydronium ion and a hydroxide ion. The equilibrium constant for the autoionization of water, KW, also known as the ion-product constant for water, has a value of 1 × 10−14. Its equilibrium expression is written as the concentration of the hydronium ions times the concentration of the hydroxide ions, with no denominator as both reactants are liquids.  In pure water at 25 °C, the concentrations of hydronium and hydroxide ions are equal—that is 1 × 10−7 M.   KW can be used to calculate the concentration of hydronium and hydroxide ions in a solution and to determine whether a solution is acidic or basic. If a solution is acidic, the concentration of hydronium ions will be higher than hydroxide ions. In contrast, if a solution is basic, the concentration of hydroxide ions will be higher than the hydronium ions. If the concentration of hydronium ions in solution is 5 × 10−7 M, the hydroxide ion concentration can be determined using KW. As the values of KW and hydronium ions are known, the concentration of hydroxide ions can be calculated to be 2 × 10−8 M. As the hydronium ion concentration, 5 × 10−7 M, is higher than the hydroxide ion concentration, 2 × 10−8 M, the solution is acidic.