A solution of two or more liquids, whose composition is not changed by distillation. The composition of the liquid phase at boiling point is identical to steam in equilibrium with it, and such mixtures or constant boiling azeotropes are solutions. The exact composition of the changes that the azeotrope of boiling point change by a change in the external pressure.

a solution of two components forming an azeotrope can be separated by distillation in a pure component and azeotrope, but not in the two pure components. Standard solutions are often prepared by distillation of aqueous solutions until the Azeotropic composition is reached. At 1 atm (760 mmHg or 100 kilopascals) pressure, hydrogen chloride and water form an azeotrope containing 20.24% HCl.