Edunes Online Education
What are Azeotropes? | Definition of Azeotropes
Chemistry | Class 11 | CBSE & SEBA Board
Azeotropes – Explained for Class 12 (CBSE Chemistry)
What is an Azeotrope?
🧪 Definition:
An azeotrope is a binary mixture that boils at a constant temperature and produces vapour with the same composition as the liquid.
This means you cannot separate the two components by simple distillation — they behave as one compound at that specific composition and boiling point.
🔍 Why does this happen?
Normally, during boiling, the composition of vapour differs from that of liquid (more volatile component escapes faster). But in an azeotropic mixture, this difference disappears at a certain fixed composition — the vapour formed has the same ratio of both components as in the liquid. Hence, no further separation by boiling is possible.
🌡️ Types of Azeotropes
Azeotropes are mainly of two types based on their boiling points in comparison to their pure components:
(i) Minimum Boiling Azeotropes
-
These mixtures boil at a lower temperature than either of the pure components.
-
They show positive deviation from Raoult’s Law (the components escape more easily due to weaker interactions).
-
The total vapour pressure is higher, so they boil at a lower temperature.
🧪 Example:
-
Ethanol (95%) + Water (5%)
-
Boiling Point ≈ 351 K (78°C)
-
This mixture forms an azeotrope that cannot be separated further by simple distillation.
🧠 Reason:
Ethanol and water form weaker hydrogen bonds in the mixture than in the pure liquids, so they escape more easily → higher vapour pressure → lower boiling point.
(ii) Maximum Boiling Azeotropes
- These mixtures boil at a higher temperature than either of the pure components.
- They show negative deviation from Raoult’s Law (the components strongly attract each other).
- The total vapour pressure is lower, so they boil at a higher temperature.
🧪 Example:
- Nitric acid (68%) + Water (32%)
- Boiling Point ≈ 393.5 K (120.5°C)
🧠 Reason:
Strong hydrogen bonding between nitric acid and water lowers the escaping tendency → lower vapour pressure → higher boiling point.
📊 Comparison Table:
| Type | Boiling Point | Deviation from Raoult's Law | Vapour Pressure | Example |
|---|---|---|---|---|
| Minimum Boiling Azeotrope | Lower than components | Positive Deviation | High | Ethanol + Water (95%) |
| Maximum Boiling Azeotrope | Higher than components | Negative Deviation | Low | Nitric Acid + Water (68%) |
🎓 Real-life Importance
-
Azeotropes limit the extent of separation by distillation.
-
Special methods like azeotropic distillation or adding third components (entrainers) are used in industries to break azeotropes.
-
Ethanol production: 95% ethanol + 5% water azeotrope is common in alcohol distillation.
💡 Analogy to Understand
Think of azeotropes as inseparable couples:
- In a normal couple (mixture), one partner might leave early (more volatile component distills first).
- In an azeotrope, both are so balanced in their interactions that they leave together, hand in hand, at the same time (same composition in vapour and liquid).
