An acid is a substance that ionises in water to form hydrogen ions (H⁺), which in solution are present as hydronium ions (H₃O⁺); their presence gives an acidic solution its characteristic properties, such as a pH below 7 and the ability to neutralise bases. Acetic acid in vinegar and hydrochloric acid in the stomach are everyday examples. [Ch.12 extension: beyond the Arrhenius (H⁺ in water) sense, an acid is also defined as a Brønsted proton donor and a Lewis electron-pair acceptor.]

酸是在水中电离生成氢离子(H⁺)的物质,这些氢离子在溶液中以水合氢离子(H₃O⁺)的形式存在;它们的存在使酸性溶液具有特征性质,如 pH 低于 7 以及能中和碱。食醋中的乙酸和胃中的盐酸都是日常例子。【第12章扩展:除阿伦尼乌斯(水中给 H⁺)定义外,酸还可定义为布朗斯特质子给体和路易斯电子对受体。】