An azeotrope is a mixture of two or more pure compounds (chemicals) in such a ratio that its composition cannot be changed by simple distillation.^[1] This is because when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture of liquids. As the composition is unchanged by boiling, azeotropes are also known as constant boiling mixtures (especially in older texts). The word azeotrope is derived from the Greek words "ζ?ειν"=boil and "τρ?πος"=change, combining with prefix "α-"=no to give the overall meaning "no change on boiling".

Types of azeotropes

Each azeotrope has a characteristic boiling point. The boiling point of an azeotrope is either less than the boiling points of any of its constituents (a positive azeotrope), or greater than the boiling point of any of its constituents (a negative azeotrope).

A well known example of a positive azeotrope is 95.6% ethanol and 4.4% water (by weight). Ethanol boils at 78.4°C, water boils at 100°C, but the azeotrope boils at 78.1°C, which is lower than either of its constituents. Indeed 78.1°C is the minimum temperature at which any ethanol/water solution can boil. It is generally true that a positive azeotrope boils at a lower temperature than any other ratio of its constituents. Positive azeotropes are also called minimum boiling mixtures.

An example of a negative azeotrope is hydrochloric acid at a concentration of 20.2% hydrogen chloride and 79.8% water (by weight). Hydrogen chloride boils at –84°C and water at 100°C, but the azeotrope boils at 110°C, which is higher than either of its constituents. Indeed 110°C is the maximum temperature at which any hydrochloric acid solution can boil. It is generally true that a negative azeotrope boils at a higher temperature than any other ratio of its constituents. Negative azeotropes are also called maximum boiling mixtures.

Deviation from Raoult's law

Raoult's law predicts the vapor pressures of ideal mixtures as a function of composition ratio. In general only mixtures of chemically similar solvents, such as n-hexane with n-heptane, form nearly ideal mixtures that come close to obeying Raoult's law. Solvent combinations that can form azeotropes are always nonideal, and as such they deviate from Raoult's law.

Total vapor pressure of mixtures as a function of composition at a chosen constant temperature.

The diagram on the right illustrates total vapor pressure of three hypothetical mixtures of constituents, X, and Y. The temperature throughout the plot is assumed to be constant.

The center trace is a straight line, which is what Raoult's law predicts for an ideal mixture. The top trace illustrates a nonideal mixture that has a positive deviation from Raoult's law, where the total combined vapor pressure of constituents, X and Y, is greater than what is predicted by Raoult's law. The top trace deviates sufficiently that there is a point on the curve where its tangent is horizontal. Whenever a mixture has a positive deviation and has a point at which the tangent is horizontal, the composition at that point is a positive azeotrope.^[6] At that point the total vapor pressure is at a maximum. Likewise the bottom trace illustrates a nonideal mixture that has a negative deviation from Raoult's law, and at the composition where tangent to the trace is horizontal there is a negative azeotrope. This is also the point where total vapor pressure is minimum.^[6]

Separation of azeotrope constituents

Distillation is one of the primary tools that chemical engineers use to separate mixtures into their constituents. Because distillation cannot separate the constituents of an azeotrope, the separation of azeotropic mixtures (also called azeotrope breaking) is a topic of considerable interest.^[2] Indeed this difficulty led some early investigators to believe that azeotropes were actually compounds of their constituents.^[1] But there are two reasons for believing that this is not the case. One is that the molar ratio of the constituents of an azeotrope is not generally the ratio of small integers. For example, the azeotrope formed by water and acetonitrile contains 2.253 moles of acetonitrile for each mole of water.^[7] A more compelling reason for believing that azeotropes are not compounds is, as discussed in the last section, that the composition of an azeotrope can be affected by pressure. Contrast that with a true compound, carbon dioxide for example, which is two moles of oxygen for each mole of carbon no matter what pressure the gas is observed at. That azeotropic composition can be affected by pressure suggests a means by which such a mixture can be separated.

Use of azeotropes to separate zeotropic mixtures

Sometimes azeotropes are useful in separating zeotropic mixtures. An example is acetic acid and water, which do not form an azeotrope. Despite this it is very difficult to separate pure acetic acid (boiling point: 118.1°C) from a solution of acetic acid and water by distillation alone.

Examples of azeotropes

Proportions are by weight.

• nitric acid (68%) / water, boils at 120.5°C at 1 atm (negative azeotrope)


• perchloric acid (28.4%) / water, boils at 203°C (negative azeotrope)


• hydrofluoric acid (35.6%) / water, boils at 111.35°C (negative azeotrope)


• ethanol (96%) / water, boils at 78.1°C


• sulfuric acid (98.3%) / water, boils at 338°C


• acetone / methanol / chloroform form an intermediate boiling (saddle) azeotrope


• diethyl ether (33%) / halothane (66%) a mixture once commonly used in anaesthesia.


• benzene / hexafluorobenzene forms a double binary azeotrope.

Source:Wikipedia

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