By John Brennan
Updated Mar 24, 2022
Boiling points vary with molecular structure. Water’s boiling point at atmospheric pressure is 100 °C (212 °F). Many gases boil far below room temperature, and some liquids—such as ethanol—also have lower boiling points than water.
Common atmospheric gases—including nitrogen (N₂), oxygen (O₂), carbon dioxide, chlorine (Cl₂) and hydrogen—boil at temperatures well below 100 °C. Liquid helium, for example, has the lowest boiling point of any substance, at about –452 °F (–268.9 °C), only 4.2 °C above absolute zero. These examples illustrate that the classification of a substance as a gas or liquid depends entirely on temperature and pressure.
Water is a polar molecule with a dipole moment; hydrocarbons such as gasoline components are nonpolar. Their intermolecular forces are dominated by London dispersion forces, which strengthen with increasing molecular size. Consequently, small nonpolar molecules typically boil at lower temperatures than water because their weaker interactions require less energy to vaporize.
Alcohols are polar and capable of forming hydrogen bonds, but they usually can form only one hydrogen bond per molecule, compared with two for water. As a result, alcohols have higher boiling points than comparable hydrocarbons but lower than water. Distillation takes advantage of this difference to concentrate ethanol in beverages like whiskey.
Ethers—compounds where an oxygen atom bridges two carbon atoms—are slightly polar but lack hydrogen‑bonding capability, giving them lower boiling points than water. Ammonia (NH₃) is another example; it boils at –33 °C and is found as a gas at room temperature, readily dissolving in water. These and other compounds further demonstrate how molecular structure dictates boiling behavior.
