CHEM 121 glossary
Boiling point
The temperature at which a liquid's vapour pressure equals the surrounding pressure, so it boils (the normal boiling point is at 1 atm). For a molecular substance it measures how strong the intermolecular forces are: stronger forces, higher boiling point.
HF breaks the size trend because it hydrogen-bonds.
Example
(18.0 g/mol) boils at 373.15 K but (16.0 g/mol) at 111.7 K: similar size, but water hydrogen-bonds and methane has only dispersion forces.