Azeotrope
Definition and meaning of Azeotrope in chemistry.
An azeotrope is a liquid mixture that boils at a constant temperature and composition. The vapor produced has the exact same chemical makeup as the starting liquid. This unique property makes azeotropes impossible to separate using normal fractional distillation methods.
In more detail
Normally, boiling a mixture turns the most volatile liquid into a gas first. This simple physical process allows chemists to separate mixed liquids quite easily. Azeotropes break this normal rule because of strong physical attractions between different molecules.
These strong physical attractions are called intermolecular forces and they change how the mixture acts. When an azeotrope boils, all the chemical components turn into gas at the exact same rate. This means the boiling liquid and the rising vapor share identical chemical ratios.
You simply cannot boil away one liquid part to leave the other liquid part behind. We classify these stubborn mixtures into two main categories based on their boiling temperatures. Sometimes the mixed molecules pull on each other more strongly than the pure molecules do.
This strong pulling creates a negative azeotrope that requires extra heat to finally boil. Other times the mixed molecules push each other away to create a positive azeotrope. Positive azeotropes always boil at a lower temperature than their pure starting parts.
Breaking a tough azeotrope requires clever chemical tricks in large industrial manufacturing plants. Engineers might add a third chemical substance to disrupt the strong molecular attractions. They might also change the surrounding air pressure to shift the mixture's boiling behavior.
Key facts
| Field | Physical Chemistry |
|---|---|
| Vapor Composition | Exactly matches the liquid composition during boiling |
| Separation limits | Cannot be separated by standard fractional distillation |
| Positive Azeotrope | Boils at a lower temperature than its pure components |
| Negative Azeotrope | Boils at a higher temperature than its pure components |
| Common Example | Ethanol and water at 95.6% ethanol concentration |
A very famous example is the positive azeotrope formed by mixing ethanol and water. Pure ethanol normally boils at a temperature of 78.4 degrees Celsius. Pure water normally boils at a much higher temperature of 100 degrees Celsius. However, a mixture containing exactly 95.6 percent ethanol boils at 78.2 degrees Celsius. This temperature is lower than the boiling points of both pure chemical parts. If you try to distill fermented alcohol, you get stuck at this exact percentage. You cannot make 100 percent pure ethanol using just a simple distillation setup.
Frequently asked questions
Why can you not separate an azeotrope by simple distillation?
The vapor rising from the boiling liquid has the exact same chemical makeup as the liquid itself. Distillation relies on changing compositions, so the process completely stops working at the azeotropic point.
What is the main difference between positive and negative azeotropes?
Positive azeotropes boil at a lower temperature than their pure chemical components. Negative azeotropes have very strong molecular attractions and boil at a much higher temperature.
How do chemical engineers separate azeotropes in the real world?
They often use a special technique called pressure-swing distillation to change the boiling point. They might also add a third chemical to break the strong molecular attractions between the liquids.