Which of the following binary mixture does not show the behaviour of minimum boiling azeotropes? 1. C₂H₅OH + C₆H₅NH₂ 2. H₂O + CH₃COC₂H₅ 3. CS₂ + CH₃COCH₃ 4. CH₃OH + CHCl₃

 

📘 Which of the Following Binary Mixtures Does Not Show the Behaviour of Minimum Boiling Azeotropes?

❓ Question:

Which of the following binary mixtures does not show the behaviour of minimum boiling azeotropes?

1. C₂H₅OH + C₆H₅NH₂

2. H₂O + CH₃COC₂H₅

3. CS₂ + CH₃COCH₃

4. CH₃OH + CHCl₃

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🔬 Understanding Azeotropes – The Basics

Before diving into the question, let’s quickly recall what azeotropes are.

✅ What are Azeotropes?

Azeotropes are liquid mixtures that boil at a constant temperature and retain the same composition in both the liquid and vapor phases. They cannot be separated by simple distillation due to this behavior.

They are broadly classified into:

• Minimum Boiling Azeotropes: Boil at a temperature lower than either of the components.

• Maximum Boiling Azeotropes: Boil at a temperature higher than either of the components.

🔁 Why Azeotropes Form?

Azeotropes occur due to non-ideal interactions between the molecules. When the attractive forces between unlike molecules are either weaker or stronger than those in pure components, it causes deviations in Raoult's law, leading to azeotropic behavior.

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💡 How to Identify Minimum Boiling Azeotropes

Minimum boiling azeotropes form when the mixture shows positive deviation from Raoult’s law. This usually happens when:

• The A–B interactions are weaker than A–A or B–B.

• The vapor pressure of the solution is higher than expected.

📊 Common Examples of Minimum Boiling Azeotropes:

Binary Mixture	Boiling Point Behavior
Ethanol + Water	Minimum boiling azeotrope
Methanol + Acetone	Minimum boiling azeotrope
Methanol + Chloroform	Minimum boiling azeotrope
Acetone + Carbon Disulfide	Minimum boiling azeotrope

📚 Explanation of Each Option

Let’s analyze each binary mixture one by one:

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✅ (1) C₂H₅OH (Ethanol) + C₆H₅NH₂ (Aniline)

• Ethanol is polar, while aniline has strong hydrogen bonding and a relatively less volatile nature.

• Aniline has strong intermolecular attractions due to its aromatic ring and NH₂ group.

• Ethanol and Aniline interact well, showing negative deviation from Raoult’s law.

• This type of interaction leads to a maximum boiling azeotrope, or sometimes no azeotrope at all.

👉 Conclusion: This does NOT show minimum boiling azeotrope behavior. ✅

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✅ (2) H₂O + CH₃COC₂H₅ (Ethyl Acetate)

• This is a polar–nonpolar combination.

• Shows positive deviation from Raoult’s law.

• The A–B interactions are weaker, leading to higher vapor pressure and minimum boiling azeotrope formation.

👉 Conclusion: This DOES form a minimum boiling azeotrope.

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✅ (3) CS₂ (Carbon Disulfide) + CH₃COCH₃ (Acetone)

• Both are volatile, non-polar or weakly polar solvents.

• Weak interactions between them cause positive deviation.

• Known to form minimum boiling azeotrope.

👉 Conclusion: This DOES form a minimum boiling azeotrope.

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✅ (4) CH₃OH (Methanol) + CHCl₃ (Chloroform)

• Chloroform can form hydrogen bonding with methanol.

• This weakens the interactions between like molecules.

• This mixture shows positive deviation, forming a minimum boiling azeotrope.

👉 Conclusion: This DOES form a minimum boiling azeotrope.

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🔍 Why Only Option (1) is Correct

The ethanol-aniline mixture shows either maximum boiling azeotrope behavior or no azeotrope at all due to strong intermolecular attractions, leading to negative deviation from Raoult’s law.

That’s why it does not behave like a minimum boiling azeotrope — and hence, it's the correct answer.

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🔬 Bonus Concept: Raoult's Law & Deviations

🔁 Raoult’s Law:

For an ideal solution:

P_total = P₁°x₁ + P₂°x₂

• Positive Deviation: A–B interactions < A–A and B–B → Forms minimum boiling azeotropes.

• Negative Deviation: A–B interactions > A–A and B–B → Forms maximum boiling azeotropes or none.

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🧠 JEE Tip & Strategy

In JEE exams, questions around azeotropes are common in organic and physical chemistry, especially in liquid mixtures or solution chapters.

🎯 Quick Tip to Crack These:

• Think about polarity: Polar + nonpolar? Likely positive deviation.

• Boiling Point: More volatile = minimum boiling azeotrope.

• Use common examples as memory anchors.

✍️ Must-Memorize Azeotrope Examples:

• Minimum Boiling:

  • Ethanol + Water

  • Methanol + Acetone

  • Acetone + CS₂

• Maximum Boiling:

• HCl + Water

• Chloroform + Acetone

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