Clausius-Clapeyron: ln(P2/P1) = -ΔHvap/R × (1/T2 - 1/T1)
A liquid boils when its vapor pressure equals the external pressure. The Clausius-Clapeyron equation predicts how boiling point changes with pressure: ln(P2/P1) = -ΔHvap/R × (1/T2 - 1/T1). Water's ΔHvap = 40.7 kJ/mol. At 0.5 atm, water boils at ~81°C; at 2 atm, ~121°C (this is how pressure cookers work). Common boiling points at 1 atm: water 100°C, ethanol 78.4°C, acetone 56.1°C, diethyl ether 34.6°C, mercury 357°C. Intermolecular forces determine BP: hydrogen bonding (water, alcohols) gives higher BPs; London dispersion forces (hydrocarbons) give lower BPs. Molecular weight within a homologous series also raises BP — each CH₂ unit adds ~20-30°C.