intermolecular attractions

Meaning: Intermolecular attractions are electrostatic forces between neighboring molecules (or between ions and molecules). They are much weaker than covalent bonds, and they are what a substance overcomes when it melts or boils; the bonds inside each molecule stay intact. The main types, roughly weakest to strongest: London dispersion forces come from temporary induced dipoles, exist in every molecule, and grow with electron count and surface area. Dipole–dipole forces act between permanently polar molecules. Hydrogen bonding occurs when an H bonded to N, O, or F is attracted to a lone pair on N, O, or F of a neighbor. Ion–dipole forces act between an ion and a polar molecule, as when Na⁺ is surrounded by water.

On the MCAT: Passages ask you to rank boiling points, melting points, vapor pressures, viscosities, or solubilities. Stronger attractions generally give a higher boiling point, lower vapor pressure, and higher surface tension. Typical comparisons: ethanol boils higher than dimethyl ether (same formula, but only ethanol hydrogen bonds), and a straight-chain alkane boils higher than its branched isomer (more surface contact, so more dispersion). Solubility follows "like dissolves like."

Common confusion: Boiling water breaks hydrogen bonds between molecules, not the O–H covalent bonds. Also, dispersion forces are not always negligible: a large nonpolar molecule can have stronger total attractions than a small polar one, so count every force present and consider size.