Why do different liquids boil at different temperatures?

Water and many other liquids can be equally warm without being equally close to boiling. Molecules in a liquid attract their neighbors, and different substances have different patterns of attraction. At the same outside pressure, these differences help determine the temperature at which vapor bubbles can persist and grow inside a liquid.

Liquid molecules are always moving. Some near the surface have enough energy to escape into the gas above: evaporation can happen below the boiling point. Other gas molecules return to the liquid. In a closed space at a steady temperature, these exchanges establish a vapor pressure. Warming the liquid generally raises that pressure because escape becomes more favorable.

A vapor bubble inside a liquid must oppose the pressure pressing inward. Surface tension adds an extra inward effect, especially for very small bubbles. If the vapor pressure is too low, a bubble collapses. At the ordinary boiling point, vapor pressure matches the outside pressure; established bubbles can then grow within the liquid. Stronger attractions between otherwise comparable molecules tend to make escape harder, so a higher temperature is needed. Lowering the outside pressure also lowers the boiling temperature; the substance has not changed into a different molecule.

All molecules can experience attractions associated with temporary fluctuations of electron distribution, called dispersion forces. Larger, more easily distorted electron clouds can produce stronger dispersion attractions. Polar molecules also have permanent uneven charge distributions. Water has particularly important hydrogen-bond attractions between molecules. These are different from the covalent O–H bonds within each water molecule. Boiling mainly overcomes attractions between molecules while leaving the molecules themselves intact.

During steady boiling at a fixed pressure, added energy can convert more liquid into vapor without continually raising the temperature. Energy is being transferred into separating molecules and expanding the vapor. Meanwhile, the exact boiling behavior of mixtures can differ from that of a pure substance. The comparison in the illustration concerns one kind of molecule at a time. A boiling point is therefore a meeting of microscopic attractions and the pressure of the surrounding world, not simply a measure of how much heat a liquid contains.