Difference Between Azeotropic and Zeotropic Mixture

The key difference between azeotropic and zeotropic mixture is that dew point and bubble point of an azeotropic mixture intersect whereas dew point and bubble point of a zeotropic mixture are distinguishable.

The terms azeotropic and zeotropic mixtures are highly related to each other since they have opposite properties to each other. Therefore, they have different dew and bubble curve characteristics as well. These dew and bubble curves are drawn in temperature-composition graphs.

CONTENTS

1. Overview and Key Difference
2. What is an Azeotropic Mixture 
3. What is a Zeotropic Mixture 
4. Side by Side Comparison – Azeotropic vs Zeotropic Mixture in Tabular Form
5. Summary

What is an Azeotropic Mixture?

Azeotropic mixture is a chemical mixture in which there are liquids having a constant boiling point. This is because the vapour of the liquid mixture has the same composition as the liquid mixture. The boiling point of this mixture can be either higher or lower than any of the individual component of the mixture.

Figure 01: Vapour-Liquid Equilibrium of 2-Propanol and Water showing Azeotropic Behaviour

Since the boiling point of the azeotropic mixture is constant, we cannot use simple distillation for the separation of components in this mixture. Therefore, we need to use some other techniques such as using two distillation columns with different degrees of separation or addition of a third compound to the azeotropic mixture in order to change the volatility and boiling point of the components.

What is a Zeotropic Mixture?

A zeotropic mixture is a mixture of liquid components having different boiling points. Since it is the opposite of an azeotropic mixture, we can call it a non-azeotropic mixture. Due to the differences in their boiling points, individual components do not undergo evaporation or condensation at the same temperature. Therefore, the mixture is in a temperature glide. The phase changes of the liquid components take place in a series of temperatures rather than at the same temperature.

Figure 02: A Temperature-Composition Graph for a Zeotropic Mixture

If we draw a temperature vs. composition graph for a zeotropic mixture, we can observe the boiling temperatures of the components between bubble point and dew point. The bubble point is the temperature at which the first bubble of vapour forms. Dew point is the temperature at which condensation takes place. The temperature vs. composition graph shows how the composition of liquid and vapour changes upon boiling, condensing and at temperatures in between them. An example of a zeotropic mixture is a mixture of ethane, methane, nitrogen, propane and isobutene.

What is the Difference Between Azeotropic and Zeotropic Mixture?

Azeotropic and zeotropic are opposite terms. The key difference between azeotropic and zeotropic mixture is that the dew point and bubble point of an azeotropic mixture intersect, whereas the dew point and bubble point of a zeotropic mixture are distinguishable. That means, we can clearly see two points as the bubble point and dew point in the graph of temperature vs. composition in zeotropic mixtures, but for an azeotropic mixture, these points lie across each other.

Following is a summary of the difference between azeotropic and zeotropic mixture.

Summary – Azeotropic vs Zeotropic Mixture

The terms azeotropic and zeotropic are opposite to each other. The key difference between azeotropic and zeotropic mixture is that the dew point and bubble point of an azeotropic mixture intersect, whereas the dew point and bubble point of a zeotropic mixture are distinguishable.

Reference:

1. “Azeotrope.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 19 Feb. 2020, Available here.
2. “Zeotropic Mixture.” Wikipedia, Wikimedia Foundation, 15 Oct. 2019, Available here.
3. “What Is an Azeotrope?” Solvent Recycling and Azeotropes, Available here.

Image Courtesy:

1. “VLEIsoropanolWaterAzeotrope” By WilfriedC – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “VLE of a Zeotropic Mixture” By WilfriedC – Own work (CC BY-SA 3.0) via Commons Wikimedia

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