Chemistry: Solutions - Lecture 4 | CHEMCA

Class 12 / JEE / NEET Chapter 1

Chemistry - Solutions | Lecture 4

By Abhishek Sengar

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Theory & Quick Notes

1 Ideal vs Non-Ideal Solutions

Solutions are categorized based on whether they obey Raoult's Law over the entire range of concentrations.

Ideal Solutions

Obeys Raoult's Law precisely.
Interactions: A-B interactions are exactly equal to A-A and B-B interactions.
ΔH_mix = 0 (No heat evolved/absorbed).
ΔV_mix = 0 (Volume is perfectly additive).
Ex: Benzene + Toluene, n-Hexane + n-Heptane

Non-Ideal Solutions

Does NOT obey Raoult's Law.
Interactions: A-B interactions differ from pure A-A and B-B interactions.
ΔH_mix ≠ 0.
ΔV_mix ≠ 0.
Ex: Chloroform + Acetone, Ethanol + Water

2 Types of Non-Ideal Deviations

Positive Deviation

The observed vapor pressure is greater than expected. This happens when the new A-B interactions are weaker than pure A-A or B-B forces, making it easier for particles to escape into vapor.

P_obs > P_exp

ΔH_mix > 0

ΔV_mix > 0

Ethanol + Acetone

Negative Deviation

The observed vapor pressure is lower than expected. This occurs when new A-B interactions are stronger (like forming new Hydrogen bonds), making it harder for particles to escape.

P_obs < P_exp

ΔH_mix < 0

ΔV_mix < 0

Acetone + Chloroform

3 Azeotropes (Constant Boiling Mixtures)

Azeotropes are binary mixtures having the same composition in both liquid and vapor phases. They boil at a constant temperature. Because the vapor composition matches the liquid, they cannot be separated by simple fractional distillation.

Minimum Boiling Azeotrope

Formed by solutions showing a large positive deviation from Raoult's law. They boil at a temperature lower than either pure component.

Example: ~95% Ethanol + Water (by volume)

Maximum Boiling Azeotrope

Formed by solutions showing a large negative deviation from Raoult's law. They boil at a temperature higher than either pure component.

Example: ~68% Nitric Acid (HNO₃) + Water

Deep Dive

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Lecture 4 Checkpoint

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