boiling point

Meaning: The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure, so vapor bubbles can form throughout the liquid. The normal boiling point is measured at 1 atm. Boiling requires enough energy to overcome intermolecular forces, so stronger attractions usually mean a higher boiling point. For molecules of similar mass, the typical ranking is hydrogen bonding (alcohols, carboxylic acids, amines) > dipole–dipole (aldehydes, ketones, esters) > London dispersion only (alkanes, alkenes). Within a family, boiling point usually rises with molecular weight and surface area, and drops with branching because branched molecules contact each other less.

On the MCAT: Passages ask you to rank compounds by boiling point, predict which component distills first in simple or fractional distillation (the lower boiling point typically comes off first), or read a graph of boiling point against carbon number. Lowering the external pressure, as in vacuum distillation, lowers the boiling point.

Common confusion: Students often rank by molar mass alone. Ethanol (46 g/mol) boils near 78 °C, while propane (44 g/mol) boils near −42 °C, because ethanol can hydrogen bond. Also, the temperature of a pure boiling liquid stays roughly constant while heat is added, because the energy goes into vaporization.