What Is a Minimum Boiling Azeotrope?

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A minimum boiling azeotrope is a liquid mixture that has a lower boiling point than its individual parts. Azeotropes, also called constant boiling mixtures, are combinations made up of two or more liquids whose chemical makeup cannot be changed by distillation. The boiling point of an azeotrope is a constant, because the vapor produced when it boils is identical in composition to the liquid mixture.

The word “azeotrope” was first used to describe this type of mixture by chemists Richard Merriman and John Wade in 1911. The word is derived from the Greek words meaning “boil,” and “turning,” and the Greek prefix meaning “no,” making the word mean literally “no turn on boiling.” Azeotropes containing two substances are called binary azeotropes. Azeotropes made up of three substances are called ternary azeotropes.

There are two types of azeotropes: minimum boiling and maximum boiling. While minimum boiling azeotropes have a boiling point that is less than any of the parts constituting the mixture, maximum boiling point azeotropes have a higher boiling point than their individual parts. Minimum boiling point azeotropes are also sometimes called positive azeotropes, while maximum boiling point azeotropes can be called negative azeotropes.

Due to their unique nature, minimum boiling azeotropes cannot be separated by simple distillation. In simple distillation, a mixture is vaporized and the material is condensed in one attempt. Because azeotropes do not have an ideal vapor-liquid curve, they undergo a process called fractional distillation. In fractional distillation, the mixture is vaporized and condensed multiple times until the components are separated into pure parts.

An example of a minimum boiling point azeotrope is 4.37% water and 95.63% ethanol. While the boiling point of water is 100 degrees Celsius and the boiling point of ethanol is 78.4 degrees Celsius, a mixture of the two boils at 78.2 degrees Celsius, lower than either of the individual substances.

79.8% water and 20.2% hydrochloric acid is an example of a maximum boiling point azeotrope. While hydrochloric acid usually boils at ?84 degrees Celsius and water boils at 100 degrees Celsius, the mixture boils at 110 degrees Celsius, a higher temperature than either of the individual boiling points.