水Water

氧原子有两对孤电子和两个 O–H 键，价层电子对呈四面体排布，H–O–H 键角 104.5°，O–H 键长 95.8 pm。氧的电负性大，分子偶极矩 1.85 D。每个水分子最多可形成四个氢键，这使水的沸点、比热容、表面张力都异常高，冰的密度也比液态水小。

Oxygen carries two lone pairs and two O–H bonds in a tetrahedral arrangement; the H–O–H angle is 104.5° and the O–H bond 95.8 pm. Oxygen's high electronegativity gives a dipole moment of 1.85 D. Each molecule can form up to four hydrogen bonds, which makes water's boiling point, heat capacity and surface tension unusually high and makes ice less dense than the liquid.

古人把水视为基本元素。1781 年卡文迪什发现氢气燃烧生成水，1783 年拉瓦锡据此证明水由氢和氧组成，并将可燃气体命名为 hydrogène（成水者）。1800 年尼科尔森和卡莱尔用伏打电堆电解水，得到体积比 2:1 的氢气和氧气。

Ancient thinkers regarded water as an element. In 1781 Cavendish found that burning hydrogen produces water, and in 1783 Lavoisier used this to show water is made of hydrogen and oxygen, naming the flammable gas hydrogène, the water-former. In 1800 Nicholson and Carlisle electrolysed water with a voltaic pile and collected hydrogen and oxygen in a 2:1 volume ratio.

水的介电常数约 80，能溶解大量离子化合物和极性分子。水会自耦电离：2 H₂O ⇌ H₃O⁺ + OH⁻，25 °C 时 Kw = 1.0 × 10⁻¹⁴。水既可作酸也可作碱，是两性物质，并参与水解、水合等大量反应。

With a dielectric constant near 80, water dissolves many ionic compounds and polar molecules. It self-ionises, 2 H₂O ⇌ H₃O⁺ + OH⁻, with Kw = 1.0 × 10⁻¹⁴ at 25 °C. It can act as acid or base, so it is amphoteric, and it takes part in hydrolysis, hydration and many other reactions.