定义Definition
氯化氢 HCl 是无色、有刺激性气味的气体，极易溶于水，其水溶液就是盐酸。氯化氢在潮湿空气中形成白雾，遇氨气生成白烟（NH₄Cl）。
Hydrogen chloride, HCl, is a colourless gas with a choking smell that is extremely soluble in water, its solution being hydrochloric acid. It forms steamy fumes in moist air and white smoke (NH₄Cl) with ammonia.
制备Preparation
实验室用浓硫酸与氯化钠加热：NaCl + H₂SO₄ → NaHSO₄ + HCl。HCl 不被浓硫酸氧化，所以能得到纯净的氯化氢；工业上由氢气在氯气中燃烧制得：H₂ + Cl₂ → 2HCl。
In the laboratory, warm sodium chloride with concentrated sulfuric acid: NaCl + H₂SO₄ → NaHSO₄ + HCl. Concentrated sulfuric acid does not oxidise HCl, so the product is pure. Industrially, hydrogen is burned in chlorine: H₂ + Cl₂ → 2HCl.
极易溶于水Very high solubility
常温下 1 体积水约能溶解 500 体积氯化氢，可做“喷泉实验”。HCl 是极性分子，溶于水时完全电离：HCl + H₂O → H₃O⁺ + Cl⁻。
At room temperature one volume of water dissolves about 500 volumes of HCl, which is shown in the 'fountain experiment'. HCl is polar and ionises completely in water: HCl + H₂O → H₃O⁺ + Cl⁻.
卤化氢的比较Comparing the hydrogen halides
热稳定性 HF > HCl > HBr > HI，因为键长增加、键焓减小；还原性则相反，HI 最强、HCl 很弱。
Thermal stability decreases HF > HCl > HBr > HI as bonds lengthen and weaken; reducing power runs the other way, HI strongest and HCl weak.
化学性质Chemical properties
与氨反应：NH₃ + HCl → NH₄Cl
With ammonia: NH₃ + HCl → NH₄Cl
与乙烯加成：CH₂=CH₂ + HCl → CH₃CH₂Cl
Adds to ethene: CH₂=CH₂ + HCl → CH₃CH₂Cl