Lessons · Bonding & structure
Same size, different boiling points: what intermolecular forces do
Methane (CH₄), ammonia (NH₃), and water (H₂O) are almost exactly the same size — 16, 17, and 18 grams per mole, three atoms of the second row wrapped around hydrogens. Yet methane boils at −161 °C, ammonia at −33 °C, and water not until +100 °C. That is a spread of more than 250 degrees between molecules of the same mass. If size isn't doing it, what is? The answer is the force acting between the molecules — and it lines up exactly with the boiling point.
| Molecule | Dominant intermolecular force | Forces present | boiling point |
|---|---|---|---|
| London dispersion | London dispersion | -161.5 °C | |
| hydrogen bonding | London dispersion, dipole–dipole, hydrogen bonding | -33.3 °C | |
| hydrogen bonding | London dispersion, dipole–dipole, hydrogen bonding | 100.0 °C |
machine-checked Line the three up by boiling point and the dominant intermolecular force climbs right alongside it: methane (London dispersion only) at the bottom, then ammonia and water (both hydrogen bonding) far higher. Stronger forces between molecules take more energy to pull apart, so the substance boils higher.
Boiling doesn't break the bonds inside a molecule — it pulls whole molecules apart from each other, against their intermolecular forces. So the boiling point measures those forces, not the covalent bonds. With size held nearly constant, methane's weak London dispersion gives way at −161 °C, while water's strong hydrogen bonding holds on until 100 °C. Water tops even ammonia because it has two O–H donors and two lone-pair acceptors — twice the hydrogen bonding. (Size and polarizability matter too, which is why dispersion alone can add up for large molecules; here we held size fixed to isolate the force.)
- ✓ Rows sorted ascending by boiling point [exact ordering]
- ✓ The dominant intermolecular force is non-decreasing in strength as the boiling point rises — IMF strength predicts the ordering [trend monotonic]
- ✓ Each row's force + boiling point re-derived from its verified molecule Atlas entry [no drift]
Boiling pulls whole molecules apart from each other — it overcomes the forces between molecules, not the covalent bonds within one (steam is still H₂O, its bonds intact). So H₂O boils at 100.0 °C not because its bonds are strong but because its intermolecular force (hydrogen bonding) is the strongest here; CH₄, about the same size, has only London dispersion and boils at -161.5 °C. The covalent bonds inside are comparably strong in both — they are simply not what boiling breaks.
Modeling assumptions — author-asserted, disclosed not discharged
- model Boiling point is being used as a stand-in for intermolecular-force strength — a fair comparison only because the three molecules are nearly the same size, so differences in London dispersion (which grows with size) are small and the dominant force is what varies.
- model The dominant intermolecular force is ranked London dispersion < dipole–dipole < hydrogen bonding for molecules of similar size. This is the standard ordering; it can invert when sizes differ a lot, since dispersion grows with molecular size and polarizability.
Concepts in this lesson
Linked into the Chemical Atlas where an entry exists; the rest fill in as the Atlas grows.
Practice this
The lesson goes deep on one scenario; the gym builds fluency by repetition. Drill these: