Azeotrope - Definition, Types, Diagram Explanation, FAQs

Azeotropes

Azeotropes are liquid mixtures of two or more components that boil at a constant temperature and have the same composition in both the liquid and vapor phases. Because the vapor produced has the same composition as the liquid mixture, the components cannot be completely separated by simple fractional distillation. Azeotropes are also known as constant-boiling mixtures. They are formed when a solution shows a significant positive or negative deviation from Raoult's Law. Depending on the nature of the deviation, azeotropes are classified into minimum-boiling azeotropes and maximum-boiling azeotropes.

Example: Ethanol–Water Mixture (Minimum Boiling Azeotrope) has approximately 95.6% ethanol and 4.4% water by mass. Boiling Point: 78.2°C, it shows positive deviation from Raoult's Law, and cannot be separated into pure ethanol and water by simple fractional distillation.

Types of Azeotropic Mixture

Azeotropic mixtures are classified into two main types based on the deviation they show from Raoult's Law.

Minimum Boiling Azeotrope (Positive Azeotrope)

A minimum boiling azeotrope shows a positive deviation from Raoult's law. In a minimum boiling azeotrope, the intermolecular forces between unlike molecules are weaker than those between like molecules. As a result, the mixture has a lower boiling point than either of its pure components.

Characteristics

• Shows positive deviation from Raoult's law.
• Boiling point is lower than that of the pure components.
• Vapour pressure is higher than expected.
• Cannot be separated by simple fractional distillation.

Examples

• Ethanol (95.6%) and water (4.4%)
• Acetone and carbon disulfide $\left(\mathrm{CS}_2\right)$

Maximum Boiling Azeotrope (Negative Azeotrope)

A maximum boiling azeotrope shows a negative deviation from Raoult's law. In a maximum boiling azeotrope, the intermolecular forces between unlike molecules are stronger than those between like molecules. Therefore, the mixture has a higher boiling point than either of the pure components.

Characteristics

• Shows negative deviation from Raoult's law.
• The boiling point is higher than that of the pure components.
• Vapour pressure is lower than expected.
• Cannot be separated by simple fractional distillation.

Examples

• Nitric acid and water
• Hydrochloric acid and water

Related topics ,

• Xenon-Difluoride
• Phosphorous Acid
• Nitrogen Dioxide
• Aluminium Sulfate
• Red Phosphorus
• Components of Air

What is a Constant Boiling Mixture

A constant boiling point mixtures are, also known as an azeotropic mixture, is a type of mixture that consists of two or more liquids whose proportions can not be altered or changed by simple distillation. According to Raoult's law, it is used to predicts the vapour pressures of an ideal mixtures with a function of composition ratio. With respect to Raoult's law, the molecules that are present within an azeotropic mixture will stick to each other to the same degree that they stick to themselves.

A constant boiling mixture (azeotrope) is a mixture of two or more liquids that boils at a constant temperature with the vapour having the same composition as the liquid.

Characteristics of Constant Boiling Mixtures

• Boil at a fixed temperature.
• Vapour and liquid have identical compositions.
• Behave like pure substances during boiling.
• Cannot be separated by simple fractional distillation.
• Form due to positive or negative deviations from Raoult's law.

Phase Diagram of Minimum and Maximum Boiling Azeotrope

Minimum-Boiling (Positive) Azeotrope
1. Shows positive deviation from Raoult's law because the attraction between unlike molecules ( $\mathrm{A}-\mathrm{B}$ ) is weaker than that between like molecules ( $\mathrm{A}-\mathrm{A}$ and $\mathrm{B}-\mathrm{B}$ ).
2. The boiling point decreases and reaches a minimum at point $\mathbf{C}$, called the azeotropic composition.
3. At point C , the liquid and vapour have the same composition ( $x_A=y_A$ ), so the mixture boils at a constant temperature.
4. The components cannot be completely separated by fractional distillation because the vapour formed has the same composition as the liquid at the azeotropic point.

Example: Ethanol-Water azeotrope.

Maximum-Boiling (Negative) Azeotrope
1. Shows negative deviation from Raoult's law because the attraction between unlike molecules (A-B) is stronger than that between like molecules ( $\mathrm{A}-\mathrm{A}$ and $\mathrm{B}-\mathrm{B}$ ).
2. The boiling point increases and reaches a maximum at point $\mathbf{D}$, known as the azeotropic composition.
3. At point $\mathbf{D}$, the liquid and vapour have identical compositions ( $x_A=y_A$ ), causing the mixture to boil at a constant temperature.
4. Fractional distillation cannot separate the mixture completely beyond the azeotropic composition since both phases have the same composition.

Example: Nitric acid-Water $\left(\mathrm{HNO}_3-\mathrm{H}_2 \mathrm{O}\right)$ azeotrope.

Also read

Some Solved Examples

Question 1: A binary liquid mixture A–B forms a minimum-boiling azeotrope at 40 mol% B. Which of the following statements is correct?

A. The mixture shows negative deviation from Raoult's law.

B. At the azeotropic composition, the vapour phase contains more B than the liquid phase.

C. The total vapour pressure of the mixture passes through a maximum.

D. The boiling point of the azeotrope is higher than those of both pure components.

Solution:

A minimum-boiling azeotrope:

• Shows positive deviation from Raoult's law.
• Has maximum vapour pressure.
• Has minimum boiling point.
• At azeotropic composition, vapour composition = liquid composition.

Hence, the correct answer is option (C)

Question 2: For a binary solution of liquids A and B , the boiling point-composition diagram exhibits a maximum at mole fraction $x_B=0.6$.

Which of the following statements is not correct?
A . The $\mathrm{A}-\mathrm{B}$ interactions are stronger than $\mathrm{A}-\mathrm{A}$ and $\mathrm{B}-\mathrm{B}$ interactions.
B. The solution exhibits negative deviation from Raoult's law.
C. At $x_B=0.6$, simple fractional distillation can completely separate A and B .
D. The total vapour pressure has a minimum at $x_B=0.6$.

Solution:

Maximum boiling point means:

• Maximum-boiling azeotrope.
• Negative deviation from Raoult's law.
• Strong A–B interactions.
• Minimum vapour pressure.
• Cannot be separated completely by distillation.

Hence, the correct answer is option (C)

Question 3: The boiling points of pure liquids A and B are 90°C and 110°C respectively. Their mixture forms a minimum-boiling azeotrope.

The boiling point of the azeotrope can be:

A. 120°C

B. 110°C

C. 100°C

D. 80°C

Solution:

A minimum-boiling azeotrope must boil at a temperature lower than both pure liquids.

$T_{\text {azeotrope }}<90^{\circ} \mathrm{C}$
Only $80^{\circ} \mathrm{C}$ satisfies this condition.

Hence, the correct answer is option (D)

Practice More Question From the Link given below: