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azeotrope
An azeotrope or a constant boiling mixture is a mixture of two or more liquids whose proportions cannot be altered by simple distillation.〔Moore, Walter J. ''Physical Chemistry'', 3rd ed., Prentice-Hall 1962, pp. 140–142〕 This happens because, when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture.
Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling mixtures. The word ''azeotrope'' is derived from the Greek words ζέειν (boil) and τρόπος (turning) combined with the prefix α- (no) to give the overall meaning, "no change on boiling". The term "azeotrope" was coined in 1911 by John Wade and Richard William Merriman.〔John Wade and Richard William Merriman (1911) "Influence of water on the boiling point of ethyl alcohol at pressures above and below the atmospheric pressure," ''Journal of the Chemical Society, Transactions'', 99 : 997-1011. From (page 1004: ) " … it is proposed, in order to avoid the cumbrous periphrase "mixtures having a minimum (or maximum) boiling point," to designate them as ''azeotropic mixtures'' (α, privative; ζέω, to boil)."〕
Many azeotropic mixtures of pairs of compounds are known,〔 and many azeotropes of three or more compounds are also known. In such a case it is not possible to separate the components by fractional distillation. There are two types of azeotropes: minimum boiling azeotrope and maximum boiling azeotrope. A solution that shows greater positive deviation from Raoult's law forms a minimum boiling azeotrope at a specific composition. For example, an ethanol-water mixture (obtained by fermentation of sugars) on fractional distillation yields a solution containing approximately 95% by volume of ethanol. Once this composition has been achieved, the liquid and vapour have the same composition, and no further separation occurs.
A solution that shows large negative deviation from Raoult's law forms a maximum boiling azeotrope at a specific composition. Nitric acid and water is an example of this class of azeotrope. This azeotrope has an approximate composition of 68% nitric acid and 32% water by mass, with a boiling point of 393.5 K.
==Types==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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