azeotrope

An azeotrope is a liquid mixture of two or more components which has a unique constant boiling point. This azeotrope may boil at a higher, lower, or intermediate temperature, relative to the constituent liquids, and the liquid retains the same composition as it is boiled. As a consequence, the vapor has the same composition as the liquid and simple distillation will not separate the constituents as it would with most liquid mixtures. The word azeotrope comes from the Greek "zein tropos", or "constant boiling". An azeotrope is said to be positive if the constant boiling point is at a temperature maximum, and negative when the boiling point is at a temperature minimum. The vast majority of azeotropes are minimum boiling. All liquid mixtures which are immiscible and which form azeotropes are minimum boiling. Examples of azeotropes :

nitric acid (68.4%) / water, boils at 122C
perchloric acid (28.4%) / water, boils at 203C (negative azeotrope)
hydrofluoric acid (35.6%) / water, boils at 111.35C (negative azeotrope)
ethanol (95%) / water, boils at 78.2C
sulphuric acid (98.3%) /water, boils at 330C
acetone / methanol / chloroform form a intermediate boiling azeotrope

Azeotrope

See Also