boiling point elevation

Meaning: A liquid boils when its vapor pressure equals the external pressure. A nonvolatile solute lowers the solvent's vapor pressure (Raoult's law) by reducing the solvent's mole fraction at the surface. The solution needs a higher temperature to reach the external pressure, so its boiling point rises.

Equation: ΔTb = i Kb m, where ΔTb is the boiling point rise (K), i is the van't Hoff factor (dimensionless, ideally the number of particles per formula unit), Kb is the solvent's molal boiling point constant (K·kg/mol), and m is molality (mol/kg).

On the MCAT: Expect a ranking question: compute i × m for each solution. At equal molality, CaCl₂ (i ≈ 3) gives a higher boiling point than NaCl (i ≈ 2), which gives a higher one than glucose (i = 1). Phase diagrams may also appear: the liquid-vapor line shifts to lower vapor pressure, so the boiling point at 1 atm moves up. Freezing point depression is the companion effect, and it lowers the freezing point.

Common confusion: Students use molarity or assume the solute's identity matters. The relationship uses molality and particle count. It assumes a nonvolatile solute in a dilute solution, and ion pairing makes the real i slightly lower than the ideal value. A volatile solute such as ethanol can lower the boiling point of a mixture with water instead of raising it.