Boiling water does not break a single O–H bond. It only pulls molecules away from each other. The weak forces between molecules decide melting points, boiling points and volatility, and they come in three strengths.
Intermolecular forces act between molecules and are much weaker than covalent bonds. Instantaneous dipole-induced dipole (id-id) forces act between all molecules: a momentary uneven electron distribution in one molecule induces a dipole in its neighbour. They grow with the number of electrons and with the contact area between molecules. Permanent dipole-permanent dipole (pd-pd) forces act as well between polar molecules, whose δ+ end of one attracts the δ− end of the next. A molecule with polar bonds is only polar if the bond dipoles do not cancel by symmetry. The strongest intermolecular force, hydrogen bonding, has its own lesson.
Switch the simulator to the id-id view and watch the electron clouds wobble. A dipole that appears in one molecule for an instant induces a matching dipole in its neighbour, and the two attract. Then compare the Group 17 elements and the noble gases.
Switch to the polarity view and look at the molecule as a whole. A molecule has a permanent dipole only if its bond dipoles add up to more than zero.
| Molecule | Polar bonds? | Shape | Polar molecule? |
|---|---|---|---|
| HCl | yes | linear, two atoms | yes |
| CO2 | yes | linear, symmetrical | no: dipoles cancel |
| CCl4 | yes | tetrahedral, symmetrical | no: dipoles cancel |
| CHCl3 | yes | tetrahedral, not symmetrical | yes |
| H2O | yes | non-linear | yes |
Polar molecules attract each other through pd-pd forces in addition to their id-id forces.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What happens when liquid bromine boils?
Instantaneous dipole-induced dipole forces are present:
Why does iodine have a higher boiling point than chlorine?
Which molecule is polar?
To compare two simple molecular substances, name every type of intermolecular force present in each, then decide which is strongest overall. Only compare like with like where the number of electrons is similar.
Never say that boiling a simple molecular substance “breaks covalent bonds”. It overcomes intermolecular forces. And do not assume the more polar molecule always boils higher: id-id forces grow so quickly with electron number that they often dominate, as HBr against HCl shows.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Pentane and 2,2-dimethylpropane are both C5H12. Why does pentane have the higher boiling point?
Propane (Mr 44) boils at −42 °C and ethanal, CH3CHO (Mr 44), at 20 °C. The best explanation is:
HCl boils at −85 °C and HBr at −67 °C. Which statement explains this?
Which substance has only instantaneous dipole-induced dipole forces between its molecules?
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