定义Definition
酸有几种定义：阿伦尼乌斯认为酸在水中电离产生 H⁺（实际为 H₃O⁺）；布朗斯特–劳里理论中酸是质子给体；路易斯理论中酸是电子对受体。酸的水溶液 pH 小于 7，有酸味，能使蓝色石蕊试纸变红。
There are several definitions of an acid: for Arrhenius, a substance that releases H⁺ (really H₃O⁺) in water; in Brønsted–Lowry theory, a proton donor; in Lewis theory, an electron-pair acceptor. Acidic solutions have pH below 7, taste sour and turn blue litmus red.
典型反应Typical reactions
酸与活泼金属反应放出氢气：Mg + 2HCl → MgCl₂ + H₂；与碱和碱性氧化物发生中和；与碳酸盐反应放出 CO₂：CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂；与氨生成铵盐。按一个分子能给出的质子数分为一元酸（HCl）、二元酸（H₂SO₄）、三元酸（H₃PO₄）。
Acids give hydrogen with reactive metals, Mg + 2HCl → MgCl₂ + H₂; neutralise bases and basic oxides; release CO₂ from carbonates, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂; and form ammonium salts with ammonia. By the protons one molecule can give they are monoprotic (HCl), diprotic (H₂SO₄) or triprotic (H₃PO₄).
强度Strength
强酸在水中完全电离，如 HCl、HNO₃、H₂SO₄；弱酸只部分电离，如乙酸、碳酸。酸的强弱用 Ka 或 pKa 衡量，与浓度无关：“强弱”和“浓稀”是两个不同的概念。
Strong acids ionise completely in water (HCl, HNO₃, H₂SO₄); weak acids only partly (ethanoic acid, carbonic acid). Strength is measured by Ka or pKa and is independent of concentration: strong/weak and concentrated/dilute are different ideas.
年表Timeline
- 1777
拉瓦锡认为一切酸都含氧，并据此给氧命名（意为“成酸元素”）
Lavoisier holds that all acids contain oxygen and names oxygen the acid-former
- 1810
戴维证明盐酸不含氧
Davy shows hydrochloric acid contains no oxygen
- 1887
阿伦尼乌斯提出电离理论
Arrhenius proposes ionic dissociation
- 1923
布朗斯特–劳里理论和路易斯理论相继提出
The Brønsted–Lowry and Lewis theories are proposed