Azeotropes are constant boiling mixtures that distil without change in composition. Minimum boiling azeotrope (positive deviation): boils at temperature lower than both pure components. Example: ethanol-water azeotrope boils at 78.1 C (vs 78.4 for ethanol, 100 for water), composition 95.5% ethanol. Maximum boiling azeotrope (negative deviation): boils above both pure components. Example: HCl-water at 108.6 C, 20.2% HCl; HNO3-water at 121 C, 68% HNO3. Azeotropes cannot be separated by simple distillation — both phases have the same composition. To break an azeotrope: use a third component (entrainer), molecular sieves, or change pressure (azeotropic composition is pressure-dependent).