Ebullioscopic Constant of Water

The mirror of freezing-point depression: dissolved particles lower the vapour pressure of the solvent, so the solution must be heated further before its vapour pressure reaches atmospheric. ΔT_b = i·K_b·m, and at 0.513 K·kg/mol the effect is nearly four times weaker than the freezing-point effect. This is why the kitchen claim that salting pasta water makes it boil hotter is technically true and practically irrelevant: a tablespoon of salt in four litres raises the boiling point by about 0.1 °C.

The constant follows from the solvent's own properties, K_b = RT_b²M/ΔH_vap, so water's high enthalpy of vaporisation is precisely what makes its value small. Solvents with weak intermolecular forces do far better: camphor's cryoscopic constant is 40 K·kg/mol, which made it the classic solvent for molar-mass determination by the Rast method. In industry the effect is a real design load in evaporators, where a concentrated liquor boils meaningfully above the temperature of pure water at the same pressure.