Compare intermolecular forces between any two substances and predict which has a higher boiling point.
Predicting which of two substances has the higher boiling point is a deceptively rich question that requires correctly identifying and ranking intermolecular forces — a core AP Chemistry Unit 3 skill, since stronger IMFs directly mean more energy (higher temperature) is required to separate molecules into the gas phase.
London dispersion forces exist between all molecules, arising from temporary, instantaneous dipoles created by random electron movement — and despite being the "weakest" force in a simplified ranking, dispersion forces actually dominate boiling point for large nonpolar molecules, since dispersion force strength increases with molecular size and surface area (more electrons, more polarizability). This is why nonpolar iodine (I2) is a solid at room temperature while nonpolar fluorine (F2) is a gas — same force type, vastly different magnitude due to size.
Dipole-dipole interactions occur between polar molecules, where the permanent partial positive end of one molecule aligns with the permanent partial negative end of another. These are generally stronger than dispersion forces between molecules of similar size, but the comparison inverts for very large nonpolar molecules versus small polar ones.
Hydrogen bonding is a particularly strong subset of dipole-dipole interaction, occurring specifically when hydrogen is bonded directly to nitrogen, oxygen, or fluorine — the three most electronegative, smallest elements capable of the tight, highly polarized N-H, O-H, or F-H bond that hydrogen bonding requires. This explains the classic exam comparison: HF (boiling point 19.5°C) boils higher than the larger, heavier HCl (boiling point -85°C) purely because HF can hydrogen bond and HCl cannot — a direct demonstration that IMF type can override molecular size.
Ethanol versus dimethyl ether is an equally revealing comparison: both share the identical molecular formula C2H6O, yet ethanol (boiling point 78°C, capable of hydrogen bonding through its O-H) boils dramatically higher than dimethyl ether (boiling point -24°C, no O-H bond, only dipole-dipole forces) — proof that molecular formula alone never determines boiling point; structure and IMF type do.
This comparator lets you select any two of 26 real molecules, identifies each substance's dominant IMF type, and predicts which has the higher boiling point with full reasoning — turning IMF ranking from a memorized rule into a checkable prediction.