📘 Definition of Azeotrope
An azeotrope is a liquid mixture that boils at a constant temperature and whose liquid and vapour phases have the same composition.
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Constant-boiling mixture
Because the vapour has the same composition as the liquid, an azeotrope cannot be separated into its components by ordinary fractional distillation.
🔥 What is a Minimum-Boiling Azeotrope?
A minimum-boiling azeotrope is a mixture that boils at a temperature lower than the boiling points of either of its pure components.
< Boiling point of A
< Boiling point of B
Such azeotropes are formed when the solution shows a positive deviation from Raoult's law.
🔬 Why does Positive Deviation occur?
In a minimum-boiling azeotrope, the attractive forces between unlike molecules A–B are weaker than the attractive forces between like molecules A–A and B–B.
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A–A and B–B attractions
Therefore, molecules escape more easily from the liquid phase into the vapour phase.
This produces a higher vapour pressure than predicted by Raoult's law.
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Greater escape tendency
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Higher vapour pressure
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Lower boiling temperature
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Minimum-Boiling Azeotrope
📌 Important Example
A common example is the mixture of ethanol and water.
The ethanol–water system forms a minimum-boiling azeotrope at approximately 95.6% ethanol by mass under atmospheric pressure. Its boiling temperature is about 78.2°C, slightly below the boiling point of pure ethanol.
Therefore, ordinary fractional distillation cannot produce completely anhydrous ethanol from this azeotropic mixture.
✅ Final Answer
Minimum-boiling azeotropes are formed due to a positive deviation from Raoult's law.
The unlike molecular interactions (A–B) are weaker than the like interactions (A–A and B–B), causing the vapour pressure to increase. Consequently, the mixture boils at a temperature lower than either pure component.
1. An azeotrope is a mixture in which:
✅ Answer
B) Liquid and vapour compositions are the same
2. A minimum-boiling azeotrope shows:
✅ Answer
A
3. In a minimum-boiling azeotrope, vapour pressure is:
✅ Answer
B
4. The boiling point of a minimum-boiling azeotrope is:
✅ Answer
B
5. Which type of intermolecular interaction produces positive deviation?
✅ Answer
B
6. A minimum-boiling azeotrope is also called:
✅ Answer
B
7. Which mixture is a well-known minimum-boiling azeotrope?
✅ Answer
A) Ethanol–water
8. Positive deviation causes the total vapour pressure to:
✅ Answer
A
9. Minimum-boiling azeotropes cannot be separated by:
✅ Answer
A
10. At the azeotropic composition:
✅ Answer
A
11. The A–B interactions in a positive deviation are generally:
✅ Answer
B
12. Higher vapour pressure generally means:
✅ Answer
B
13. Minimum-boiling azeotrope occurs at a composition where:
✅ Answer
B
14. Which law describes ideal liquid-solution behaviour?
✅ Answer
B
15. A minimum-boiling azeotrope has a boiling point that is:
✅ Answer
A
16. Which deviation is associated with weaker intermolecular forces?
✅ Answer
A
17. A minimum-boiling azeotrope is generally:
✅ Answer
B
18. Ethanol–water forms an azeotrope because of:
✅ Answer
B
19. The major consequence of azeotrope formation is:
✅ Answer
B
20. Which statement is correct?
✅ Answer
B) Minimum-boiling azeotropes show positive deviation
Q. Why do certain liquid mixtures form minimum-boiling azeotropes?
Solution: Minimum-boiling azeotropes are formed due to positive deviation from Raoult's law. The attractive forces between unlike molecules are weaker than those between like molecules, causing higher vapour pressure and consequently a lower boiling point.
Q. Explain the formation of a minimum-boiling azeotrope.
Solution:
- The mixture shows positive deviation from Raoult's law.
- A–B intermolecular attractions are weaker than A–A and B–B attractions.
- The vapour pressure becomes higher and the boiling point becomes lower than that of either pure component.
Hence, a minimum-boiling azeotrope is formed.
Q. Explain the relationship between positive deviation and minimum-boiling azeotrope.
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Weaker A–B attraction
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Higher Vapour Pressure
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Lower Boiling Point
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Minimum-Boiling Azeotrope
Thus, a minimum-boiling azeotrope occurs when the total vapour pressure shows a maximum and the boiling temperature shows a minimum at a particular composition.
Q. Explain minimum-boiling azeotropes with an example.
Solution: A minimum-boiling azeotrope is a constant-boiling mixture that boils at a temperature lower than the boiling points of both pure components.
It is formed when the solution shows positive deviation from Raoult's law. The A–B attractions are weaker than A–A and B–B attractions. Consequently, molecules escape more readily from the liquid phase, increasing the vapour pressure.
Since boiling occurs when vapour pressure becomes equal to external pressure, the higher vapour pressure causes the solution to boil at a lower temperature.
Example: Ethanol–water forms a minimum-boiling azeotrope under atmospheric pressure.
Q. Explain in detail why certain liquid mixtures form minimum-boiling azeotropes.
1. Azeotrope
An azeotrope is a constant-boiling mixture in which the composition of the vapour phase is the same as that of the liquid phase at the azeotropic composition.
2. Positive Deviation from Raoult's Law
Minimum-boiling azeotropes are associated with positive deviation from Raoult's law. The observed vapour pressure is greater than the vapour pressure predicted for an ideal solution.
3. Weak A–B Interactions
The intermolecular attractions between unlike molecules, A–B, are weaker than the attractions between like molecules, A–A and B–B.
4. Increase in Vapour Pressure
Because the A–B attractions are weaker, molecules can escape from the liquid phase more easily. Therefore, the vapour pressure becomes relatively high.
5. Decrease in Boiling Point
A liquid boils when its vapour pressure becomes equal to the external pressure. Since the vapour pressure is higher, the mixture reaches the boiling condition at a lower temperature.
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Positive deviation
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Higher vapour pressure
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Lower boiling temperature
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Minimum-boiling azeotrope
6. Example
The ethanol–water system is a well-known example of a minimum-boiling azeotrope under atmospheric pressure.
Certain liquid mixtures form minimum-boiling azeotropes because of positive deviation from Raoult's law, caused by weaker unlike-molecule interactions. This gives a higher vapour pressure and hence a lower boiling temperature at the azeotropic composition.
🎯 Quick Revision
A–B interaction → Weaker
Vapour pressure → Higher
Boiling point → Lower
Example → Ethanol–water
Separation by ordinary fractional distillation → Not possible at azeotropic composition
