Azeotrope

I'm having trouble understanding what an azeotrope is. Any clarification is appreciated.

Sheryl

Two liquids with different boiling points normally can be separated by fractional distillation; i.e., the lower boiling point material boils first and when all of it has been evaporated and condensed (and separated into another container), then the temperature is raised until the second material begins to boil. This is another way of saying that the liquid phase and the vapor phase of the two compounds is very different. The vapor phase consists primarily of the lower boiling point material while the liquid phase consists of say equal parts of the two materials. However, some materials form azeotropes; that is, the composition in the vapor phase and the composition of the liquid phase is the same. An ethyl alcohol-water mixture forms an azeotrope which is about 95% EtOH and 5% H2O. When this occurs, there is no separation because the liquid phase is 95/5 and the vapor phase is 95/5; therefore, the 95/5 mixture is what boils and the same mixture then is condensed. HCl/H2O forms an azeotrope, also, and is so reliable that it can be used as a standard. It is approximately 22%HCl/78%H2O.

You said the composition in the vapor phase and the composition of the liquid phase is the same. How about if the liquid is 40% water and 60% ethanol. Does this mean that the vapor is 40% water? How can that be if the azeotrope is 95% ethanol? It sounds like the vapor phase is steady at a ratio of 95:5.

Thanks for any clarification.

Sheryl

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