Dipole-dipole forces

Meaning: A polar molecule has a permanent dipole because of polar bonds arranged asymmetrically, so one end is δ+ and the other δ−. In a liquid or solid, molecules orient so opposite partial charges are near each other, giving a net attraction. This is one type of intermolecular force (between molecules), not a bond within a molecule.

On the MCAT: You are usually asked to rank boiling points, melting points, vapor pressures or solubility. For molecules of similar molar mass, the more polar one has stronger dipole-dipole forces, a higher boiling point and a lower vapor pressure. Typical order of strength for similar-sized molecules: London dispersion < dipole-dipole < hydrogen bonding < ion-dipole. Also expect "like dissolves like": polar solutes dissolve in polar solvents because dipole-dipole attractions replace those lost. Predicting polarity requires drawing the geometry: CO₂ has polar bonds but no net dipole, while H₂O and SO₂ do.

Common confusion: Students assume that polar bonds always make a polar molecule. Symmetry can cancel bond dipoles (CCl₄, CO₂, BF₃), leaving only dispersion forces. A second trap is assuming dipole-dipole always beats dispersion. Dispersion forces grow with molar mass and polarizability, so a very large nonpolar molecule can have a higher boiling point than a small polar one. Compare molecules of similar size first.