Editing Azeotrope (section)

== Miscibility and zeotropy  ==
[[File:HeteroazeotropePhaseDiagram.svg|thumb|left|Phase diagram illustrating how the various phases of a heteroazeotrope are related.<ref name="moore" /><ref name="hilmen">{{Cite web |last=Hilmen |first=Eva-Katrine |date=November 2000 |title=Separation of Azeotropic Mixtures: Tools for Analysis and Studies on Batch Distillation Operation |url=http://www.chemeng.ntnu.no/thesis/download/2000/hilmen/Thesis_Hilmen.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070615185910/http://www.chemeng.ntnu.no/thesis/download/2000/hilmen/Thesis_Hilmen.pdf |archive-date=15 June 2007 |access-date=24 March 2007 |publisher=Norwegian University of Science and Technology, Dept. of Chemical Engineering}}</ref> Vertical axis is temperature, horizontal axis is composition. The dotted vertical line indicates the composition of the combined layers of the distillate whenever both layers are present in the original mixture]]

If the constituents of a mixture  are completely [[miscible]] in all proportions with each other, the type of azeotrope is called a ''homogeneous azeotrope''. Homogeneous azeotropes can be of the low-boiling or high-boiling azeotropic type. For example, any amount of ethanol can be mixed with any amount of water to form a homogeneous solution.

If the components of a mixture are not completely miscible, an azeotrope can be found inside the [[miscibility gap]]. This type of azeotrope is called a ''heterogeneous azeotrope'' or [[heteroazeotrope]].  A heteroazeotropic distillation will have two liquid phases. Heterogeneous azeotropes are only known in combination with temperature-minimum azeotropic behavior. For example, if equal volumes of [[chloroform]] (water solubility 0.8 g/100 ml at 20&nbsp;°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&nbsp;°C, which is lower than either the boiling point of chloroform (61.2&nbsp;°C) or the boiling point of water (100&nbsp;°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.<ref>''CRC Handbook of Chemistry and Physics'', 44th ed., p. 2156.</ref>

Combinations of solvents that do not form an azeotrope when mixed in any proportion are said to be [[Zeotropic mixture|zeotropic]]. Azeotropes are useful in separating zeotropic mixtures. An example is zeotropic [[acetic acid]] and water. It is very difficult to separate out pure acetic acid (boiling point: 118.1&nbsp;°C): progressive distillations produce drier solutions, but each further distillation becomes less effective at removing the remaining water. Distilling the solution to dry acetic acid is therefore economically impractical. But [[ethyl acetate]] forms an azeotrope with water that boils at 70.4&nbsp;°C. By adding ethyl acetate as an entrainer, it is possible to distill away the azeotrope and leave nearly pure acetic acid as the residue.