沸点Boiling point

分子物质沸腾时克服的是分子间作用力，而不是分子内的共价键。分子间作用力越强，沸点越高。对结构相似的分子，相对分子质量越大、电子越多，范德华力越强，沸点越高，如卤素单质 F₂ < Cl₂ < Br₂ < I₂。能形成氢键的物质沸点反常地高，例如 H₂O（100 °C）远高于 H₂S（−60 °C）。

When a molecular substance boils, intermolecular forces are overcome, not the covalent bonds within molecules. The stronger the intermolecular forces, the higher the boiling point. For similar molecules, more electrons (larger Mr) mean stronger van der Waals forces and higher boiling points, as in F₂ < Cl₂ < Br₂ < I₂. Substances that hydrogen-bond boil unusually high: H₂O (100 °C) far above H₂S (−60 °C).

同分异构体中，支链越多，分子越接近球形，分子间接触面积越小，范德华力越弱，沸点越低。例如 C₅H₁₂ 的三种异构体：正戊烷 36 °C，2-甲基丁烷 28 °C，2,2-二甲基丙烷 10 °C。

Among isomers, more branching makes the molecule more spherical, reducing the contact area between molecules and weakening van der Waals forces, so the boiling point falls. The three isomers of C₅H₁₂: pentane 36 °C, 2-methylbutane 28 °C, 2,2-dimethylpropane 10 °C.

外压降低，沸点降低：高原上水在不到 100 °C 时就沸腾，减压蒸馏可在较低温度下蒸出易分解的物质。外压升高则沸点升高，高压锅利用的正是这一点。溶有难挥发溶质的溶液蒸气压降低，沸点升高，升高值与溶质粒子浓度成正比，属于依数性。

Lower external pressure lowers the boiling point: at altitude water boils below 100 °C, and vacuum distillation removes heat-sensitive substances at lower temperatures. Higher pressure raises it, which is how a pressure cooker works. Dissolving an involatile solute lowers the vapour pressure and raises the boiling point in proportion to the concentration of solute particles, a colligative property.