Azeotropes – Explained for Class 12 (CBSE Chemistry)

 

📘 Azeotropes – Explained for Class 12 (CBSE Chemistry)

🔹 What is an Azeotrope?

An azeotrope is a mixture of two (or more) liquids that boils at a constant temperature and behaves like a pure substance during boiling.

🧪 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).