Heteroazeotropes
In each of the examples discussed so far the constituents have been miscible in all proportions with each other. For example, any amount of ethanol can be mixed with any amount of water to form a homogeneous solution. There are pairs of solvents for which this is not the case. For example, if equal volumes of chloroform (water solubility 0.8 g/100 ml at 20°C) and water are shaken together and then left to stand, the liquid will separate into two layers. Analysis of the layers shows that the top layer is mostly water with a small amount of chloroform dissolved in it, and the bottom layer is mostly chloroform with a small amount of water dissolved in it. If the two layers are heated together, the system of layers will boil at 53.3°C, which is lower than either the boiling point of chloroform (61.2°C) or the boiling point of water (100°C). The vapor will consist of 97.0% chloroform and 3.0% water regardless of how much of each liquid layer is present (provided both layers are indeed present). If the vapor is re-condensed, the layers will reform in the condensate, and will do so in a fixed ratio, which in this case is 4.4% of the volume in the top layer and 95.6% in the bottom layer. Such a system of solvents is known as a heteroazeotrope. The diagram illustrates how the various phases of a heteroazeotrope are related.
Heteroazeotropes are always minimum boiling mixtures.
Read more about this topic: Azeotrope