What Sets a Boiling Point?

A liquid boils when its molecules gain enough energy to escape one another, so the boiling point measures how strongly the molecules attract each other. Stronger intermolecular forces = higher boiling point.

The Three Forces

How to Compare Two Compounds

First compare the forces: at a similar molecular weight, hydrogen bonding beats dipole–dipole, which beats dispersion alone. If the two compounds have the same functional groups, compare size: more carbons means more dispersion. If they are isomers, compare shape: branching pulls the skeleton toward a sphere and shrinks the contact surface, so the straighter isomer boils higher.

Note that a strong force can outweigh a large mass difference: water (18 amu) boils 100 °C above butane (58 amu).

Which Boils Higher?

The compound with the stronger intermolecular forces boils higher: compare 1) hydrogen bonding, 2) dipole–dipole attraction, and 3) dispersion, then check the measured values. All boiling points here are experimental values at atmospheric pressure.

Pick the compound with the higher boiling point. When the pair has a clean explanation, a follow-up asks you why.

Compound A

Compound B

Loading...

Which boils higher?

I don't know

Streak: 0 Best: 0

Common Questions

What determines a compound's boiling point?

A liquid boils when its molecules gain enough energy to escape each other, so boiling point measures the strength of the intermolecular forces: hydrogen bonding is strongest, then dipole–dipole attraction, then dispersion forces. Dispersion grows with molecular size and surface contact, so both the type of force and the size of the molecule matter.

Why does branching lower the boiling point?

Branching pulls a carbon skeleton toward a sphere, which shrinks the surface two molecules can press together, and dispersion forces act through that contact surface. Pentane boils at 36 °C while its branched isomer neopentane boils at 9.5 °C — same formula, less contact.

Why does water boil so high for its size?

Water is 18 amu and boils at 100 °C; butane is 58 amu and boils at 0 °C. Each water molecule can donate two hydrogen bonds and accept two more, and that network outweighs three times its mass in dispersion forces.

Do polar molecules always boil higher than nonpolar ones?

No. The comparison only works at similar molecular weight, and there are real exceptions even then: perfluorinated compounds carry many polar C–F bonds, yet the bond dipoles cancel and fluorine holds its electrons so tightly that dispersion is weak. Hexane (86 amu) boils 12 °C higher than perfluorohexane (338 amu).