Read Which Force Actually Dominates Before Ranking Attractions
Ranking intermolecular attractions means identifying the STRONGEST force actually available in each option — ion-dipole beats hydrogen bonding, which beats dipole-dipole, which beats dispersion (London) forces, roughly in that order.
Motion and force questions become simpler when direction, starting condition, time and the acting forces are marked first.
Intramolecular vs intermolecular hydrogen bonding changes physical properties oppositely
o-Hydroxybenzoic acid (salicylic acid) forms an intramolecular hydrogen bond within one molecule, which LOWERS its boiling/melting point relative to the para isomer — because that hydrogen bond is 'used up' internally and can't also link separate molecules together the way intermolecular H-bonds do in the para isomer.
Hydrogen bonding explains real anomalies in solubility and volatility
Ammonia's high water solubility (versus phosphine's near-insolubility) and HF's uniquely low volatility among the hydrogen halides both trace back to hydrogen bonding: N-H and H-F bonds are polar and small enough to hydrogen-bond effectively, while P-H is not polar/small enough, and Cl/Br/I are too large and insufficiently electronegative for HCl/HBr/HI to hydrogen-bond the same way.
Ranking attraction strength across different force types
When comparing genuinely different force types in one question (ion-dipole vs dipole-dipole vs dispersion), ion-dipole is strongest whenever an actual ion is present — don't let a familiar-looking dipole-dipole pairing distract from a weaker ion-dipole option elsewhere in the choices, or vice versa; always identify which force TYPE each option actually represents first.
OlympIQ exam lens
- ✓Force strength ranking (strongest to weakest, roughly): ion-dipole > hydrogen bonding > dipole-dipole > dispersion (London) forces.
- ✓Intramolecular hydrogen bonding lowers boiling/melting point relative to an isomer that forms intermolecular hydrogen bonds instead.
- ✓N-H and H-F hydrogen-bond effectively (explaining NH3's water solubility and HF's low volatility); P-H and the heavier hydrogen halides largely don't.
- ✓Identify the force TYPE in each option before ranking — don't assume the most 'familiar-looking' pairing is automatically the strongest.
Don't assume hydrogen bonding is always present just because H and an electronegative atom both appear in the molecule — P-H, C-H and S-H bonds are generally too weakly polar (or the atom too large/insufficiently electronegative) to hydrogen-bond the way N-H, O-H and F-H do.
Ready to try the pattern?
100% coverage across the 12 linked questions.
Practise 12 linked questions