Chemists often gain insight into physical properties through examination of trends in closely related molecules. Explain the trend in the boiling points of the hydrogen halides: HCl, 285 °C; HBr, 267 °C; HI, 25 °C.

PLAN Stronger intermolecular forces result in higher boiling points. The dipole moments, and therefore the strength of the dipole–dipole interactions, increase with the polarity of the H—X bond and therefore with the differences in electronegativity between hydrogen and the halogens. The strength of London forces increases with the number of electrons. Use the data to identify the dominant effect.