This lesson explains how substances exist in different phases, how phase changes occur, and how vapor pressure and Raoult’s Law govern the behavior of ideal and non‑ideal solutions.

1. Core Concepts (Short Notes)

18.1 Phase and Phase Equilibrium

A phase is a physically and chemically uniform part of a system. Examples: solid, liquid, gas.

Phase equilibrium occurs when:

• A substance exists in two phases (e.g., liquid ↔ vapor).

• Rate of evaporation = rate of condensation.

• Vapor pressure becomes constant.

18.2 Vapor Pressure

The pressure exerted by vapor molecules in equilibrium with their liquid.

• Increases with temperature.

• Depends on intermolecular forces (strong forces → low vapor pressure).

18.3 Boiling Point

A liquid boils when its vapor pressure equals external pressure.

• Lower external pressure → lower boiling point.

• Higher external pressure → higher boiling point.

18.4 Phase Diagram

Graph showing phases at different temperatures and pressures. Key points:

• Triple point: All 3 phases coexist.

• Critical point: Beyond this, liquid & gas phases become indistinguishable.

2. Solutions & Raoult’s Law

18.5 Ideal Solutions

A solution obeys Raoult’s Law when:

• Interactions between A–B are similar to A–A and B–B.

• No heat change on mixing.

• No volume change on mixing.

18.6 Raoult’s Law

For component A in an ideal solution: [ P_A = X_A P_A^0 ] Where:

• ( P_A ): partial vapor pressure of A

• ( X_A ): mole fraction of A

• ( P_A^0 ): vapor pressure of pure A

Total pressure: [ P_{total} = P_A + P_B ]

18.7 Non‑Ideal Solutions

Deviation from Raoult’s Law occurs when intermolecular forces differ.

• Positive deviation: weaker interactions → higher vapor pressure.

• Negative deviation: stronger interactions → lower vapor pressure.

Examples:

• Positive deviation: ethanol + acetone.

• Negative deviation: chloroform + acetone.

18.8 Azeotropes

Mixtures that boil at constant temperature and behave like pure substances.

• Minimum‑boiling: positive deviation.

• Maximum‑boiling: negative deviation.

3. Key Formulas to Memorize

• Raoult’s Law: ( P_A = X_A P_A^0 )

• Total Pressure: ( P = X_A P_A^0 + X_B P_B^0 )

• Mole Fraction: ( X_A = n_A / (n_A + n_B) )

4. Tips & Tricks for Exams

• Check if the solution is ideal: similar polarity → more ideal.

• Higher vapor pressure = lower boiling point.

• For multi‑component mixtures, calculate partial pressures separately.

• Identify deviations by comparing experimental vs. predicted pressure.

• Azeotropes appear as minima or maxima on boiling point graphs.

5. Important Points to Remember

• Vapor pressure increases with temperature.

• Boiling occurs when vapor pressure equals external pressure.

• Raoult’s Law applies to ideal solutions only.

• Non‑ideal solutions show positive or negative deviation.

Azeotropes cannot be separated by simple distillation.