An azeotrope is a mixture of two or more components which has a constant boiling point at a particular composition. A consequence of this constant boiling point is that one cannot entirely separate the components by distillation (though of course, in the case of the ethanol-water azeotrope, you can get close to a pure product).

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.

Examples of azeotropes :

  • nitric acid (68.4%) / water, boils at 122C
  • perchloric acid (28.4%) / water, boils at 203C (negative azeotrope)
  • fluorhydric acid (35.6%) / water, boils at 111.35C (negative azeotrope)
  • ethanol (95%) / water, (boils at x %)
  • sulphuric acid (98.3%) /water, boils at 330C