Azeotropes and Azeotropic Mixtures:- A mixture of two miscible liquids can be separated by the method of fractional distillation due to difference in their boiling points. But at a particular composition of two liquids, the mixture of liquids (solution) boils like a pure liquid and cannot be separated by fractional distillation. Such solution are called Azeotropic mixtures or Azotorpe.

The Azeotropes are defined as, “The mixture of liquid which boils at constant temperature like a pure liquid and possess same composition of components in liquid as well as in vapour phase.” These are also called the constant boiling mixtures. e.g. a solution of 88% HNO₃ (B.Pt 359K) and 32% H₂O (B.pt 373 K) boils at 393 K like a pure liquid.

There are certain properties of dilute solutions (ideal solutions) which depend upon the number of particles (molecules, ions and atom) of solute i.e. molar concentration of solute and not upon the nature of solute, these are called Colligative Properties. Thus colligative properties may be defined as “Those properties of ideal solutions (dilute solution) which depend only on the number of moles of solute (solute particles) and not upon the nature of solute. “The four important Colligative properties are:-

1. Relative lowering of vapour pressure
2. Osmotic pressure.
3. Elevation in Boiling Point.
4. Depression in freezing point.