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Chem

An azeotrope is a liquid mixture that boils at a constant temperature with the same composition in both liquid and vapor phases, making it impossible to separate its components through fractional distillation. Deviations from Raoult's law can lead to positive deviations, resulting in minimum-boiling azeotropes, or negative deviations, leading to maximum-boiling azeotropes. Examples include ethanol and water forming a minimum-boiling azeotrope, and HCl and water forming a maximum-boiling azeotrope.

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7 views2 pages

Chem

An azeotrope is a liquid mixture that boils at a constant temperature with the same composition in both liquid and vapor phases, making it impossible to separate its components through fractional distillation. Deviations from Raoult's law can lead to positive deviations, resulting in minimum-boiling azeotropes, or negative deviations, leading to maximum-boiling azeotropes. Examples include ethanol and water forming a minimum-boiling azeotrope, and HCl and water forming a maximum-boiling azeotrope.

Uploaded by

samriddhivishy
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Alright, let’s break this down clearly.

What is an azeotrope?

●​ An azeotrope is a liquid mixture of two (or more) substances that boils at a constant
temperature and has the same composition in both liquid and vapor phase.​

●​ This means you cannot separate its components by simple fractional


distillation, because the vapor you collect has the same ratio of components as the
liquid you started with.​

●​ Azeotropes are often called constant boiling mixtures.​

Connection to deviations from Raoult’s law

Raoult’s law predicts how the vapor pressure of a solution behaves ideally. Real solutions
can deviate:

1.​ Positive Deviation (higher vapor pressure than expected)​

○​ Happens when A–B intermolecular forces are weaker than A–A or B–B
forces.​

○​ Molecules escape more easily → higher vapor pressure → lower boiling


point.​

○​ If the deviation is large enough, the mixture forms a minimum-boiling


azeotrope (boils at a lower temperature than either pure component).​

○​ Example: Ethanol + Water (azeotrope at ~95% ethanol, boils at 78.1°C, lower


than pure ethanol).​

2.​ Negative Deviation (lower vapor pressure than expected)​

○​ Happens when A–B intermolecular forces are stronger than A–A or B–B
forces.​

○​ Molecules are held tightly → lower vapor pressure → higher boiling point.​
○​ If the deviation is large enough, the mixture forms a maximum-boiling
azeotrope (boils at a higher temperature than either pure component).​

○​ Example: HCl + Water (azeotrope at ~20% HCl, boils at 110°C, higher than
either component).​

✅ In short:
●​ Azeotrope = constant boiling mixture (liquid = vapor composition).​

●​ Positive deviation → minimum-boiling azeotrope.​

●​ Negative deviation → maximum-boiling azeotrope.​

Do you want me to also make a simple diagram showing Raoult’s law curves, deviations,
and azeotrope points? That makes it click instantly.

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