Ideal and Non-Ideal Solutions
Deviations from Raoult's law, azeotropes, and why like dissolves like
When ethanol and water are mixed, the total volume is slightly less than the sum of the pure volumes — the mixture "contracts." When acetone and carbon disulphide are mixed, the volume expands slightly. What do these observations reveal about the intermolecular forces in each mixture — and what would you predict about how their boiling points compare to Raoult's law predictions?
Pure ethanol boils at 78.4°C. Pure water boils at 100°C. You might expect that a mixture would boil somewhere between these two temperatures. But at 95.6% ethanol, the mixture boils at 78.1°C — lower than pure ethanol. This is a minimum-boiling azeotrope: a mixture that boils at a lower temperature than either pure component. You can't distil it any further — the vapour has the same composition as the liquid. This is why you cannot make 100% pure alcohol by distillation alone. Every bottle of whisky, rum, and vodka is a living demonstration of non-ideal solution behaviour.
Ideal Solutions
An ideal solution obeys Raoult's law at every concentration. It forms when:
- A-B intermolecular forces = A-A and B-B forces
- (no heat released or absorbed on mixing)
- (no volume change on mixing)
Examples: benzene + toluene, ethanol + methanol, -hexane + -heptane (similar molecules, similar forces)
Non-Ideal Solutions — Deviations from Raoult's Law
Positive vs Negative Deviation
Positive Deviation
- p_observed > p_Raoult (higher vapour pressure than predicted)
- A-B interactions weaker than A-A and B-B
- ΔH_mix > 0 (endothermic mixing)
- ΔV_mix > 0 (volume expansion)
- Forms minimum-boiling azeotrope
- Examples: ethanol-water, acetone-CS₂, acetone-benzene
Negative Deviation
- p_observed < p_Raoult (lower vapour pressure than predicted)
- A-B interactions stronger than A-A and B-B
- ΔH_mix < 0 (exothermic mixing)
- ΔV_mix < 0 (volume contraction)
- Forms maximum-boiling azeotrope
- Examples: acetone-chloroform, HNO₃-water, phenol-aniline
Positive Deviation
- p_observed > p_Raoult (higher vapour pressure than predicted)
- A-B interactions weaker than A-A and B-B
- ΔH_mix > 0 (endothermic mixing)
- ΔV_mix > 0 (volume expansion)
- Forms minimum-boiling azeotrope
- Examples: ethanol-water, acetone-CS₂, acetone-benzene
Negative Deviation
- p_observed < p_Raoult (lower vapour pressure than predicted)
- A-B interactions stronger than A-A and B-B
- ΔH_mix < 0 (exothermic mixing)
- ΔV_mix < 0 (volume contraction)
- Forms maximum-boiling azeotrope
- Examples: acetone-chloroform, HNO₃-water, phenol-aniline
Azeotropes
An azeotrope is a mixture that boils at a constant temperature and has the same composition in vapour and liquid phases — it cannot be further separated by simple distillation.
Minimum-boiling azeotrope (positive deviation):
- Boils below both pure components
- Example: ethanol-water at 95.6% ethanol, 78.1°C
- Absolute alcohol cannot be made by distillation alone
Maximum-boiling azeotrope (negative deviation):
- Boils above both pure components
- Example: -water at 68% , 120.5°C
- Purifying by distillation produces the azeotropic composition, not pure component
AI Generation Prompt
Two side-by-side vapour pressure vs mole fraction graphs. Left graph labelled 'Positive Deviation': X-axis mole fraction of A (0 to 1), Y-axis vapour pressure. Show dashed straight Raoult's law line between p°_B and p°_A. Show solid curve bulging ABOVE the dashed line, labelled 'Observed p_total'. Label region 'p > Raoult prediction'. Right graph labelled 'Negative Deviation': same axes. Show dashed straight Raoult's law line. Show solid curve dipping BELOW the dashed line, labelled 'Observed p_total'. Label region 'p < Raoult prediction'. Both graphs show component contributions p_A and p_B. Dark background, orange accent labels, clean technical illustration style.
Q1.Which conditions must both be satisfied for a solution to be ideal?
When ethanol and water are mixed, the total volume is slightly less than the sum of the pure volumes — the mixture "contracts." When acetone and carbon disulphide are mixed, the volume expands slightly. What do these observations reveal about the intermolecular forces in each mixture — and what would you predict about how their boiling points compare to Raoult's law predictions?
Pure ethanol boils at 78.4°C. Pure water boils at 100°C. You might expect that a mixture would boil somewhere between these two temperatures. But at 95.6% ethanol, the mixture boils at 78.1°C — lower than pure ethanol. This is a minimum-boiling azeotrope: a mixture that boils at a lower temperature than either pure component. You can't distil it any further — the vapour has the same composition as the liquid. This is why you cannot make 100% pure alcohol by distillation alone. Every bottle of whisky, rum, and vodka is a living demonstration of non-ideal solution behaviour.
Ideal Solutions
An ideal solution obeys Raoult's law at every concentration. It forms when:
- A-B intermolecular forces = A-A and B-B forces
- (no heat released or absorbed on mixing)
- (no volume change on mixing)
Examples: benzene + toluene, ethanol + methanol, -hexane + -heptane (similar molecules, similar forces)
Non-Ideal Solutions — Deviations from Raoult's Law
Positive vs Negative Deviation
Positive Deviation
- p_observed > p_Raoult (higher vapour pressure than predicted)
- A-B interactions weaker than A-A and B-B
- ΔH_mix > 0 (endothermic mixing)
- ΔV_mix > 0 (volume expansion)
- Forms minimum-boiling azeotrope
- Examples: ethanol-water, acetone-CS₂, acetone-benzene
Negative Deviation
- p_observed < p_Raoult (lower vapour pressure than predicted)
- A-B interactions stronger than A-A and B-B
- ΔH_mix < 0 (exothermic mixing)
- ΔV_mix < 0 (volume contraction)
- Forms maximum-boiling azeotrope
- Examples: acetone-chloroform, HNO₃-water, phenol-aniline
Positive Deviation
- p_observed > p_Raoult (higher vapour pressure than predicted)
- A-B interactions weaker than A-A and B-B
- ΔH_mix > 0 (endothermic mixing)
- ΔV_mix > 0 (volume expansion)
- Forms minimum-boiling azeotrope
- Examples: ethanol-water, acetone-CS₂, acetone-benzene
Negative Deviation
- p_observed < p_Raoult (lower vapour pressure than predicted)
- A-B interactions stronger than A-A and B-B
- ΔH_mix < 0 (exothermic mixing)
- ΔV_mix < 0 (volume contraction)
- Forms maximum-boiling azeotrope
- Examples: acetone-chloroform, HNO₃-water, phenol-aniline
Azeotropes
An azeotrope is a mixture that boils at a constant temperature and has the same composition in vapour and liquid phases — it cannot be further separated by simple distillation.
Minimum-boiling azeotrope (positive deviation):
- Boils below both pure components
- Example: ethanol-water at 95.6% ethanol, 78.1°C
- Absolute alcohol cannot be made by distillation alone
Maximum-boiling azeotrope (negative deviation):
- Boils above both pure components
- Example: -water at 68% , 120.5°C
- Purifying by distillation produces the azeotropic composition, not pure component
AI Generation Prompt
Two side-by-side vapour pressure vs mole fraction graphs. Left graph labelled 'Positive Deviation': X-axis mole fraction of A (0 to 1), Y-axis vapour pressure. Show dashed straight Raoult's law line between p°_B and p°_A. Show solid curve bulging ABOVE the dashed line, labelled 'Observed p_total'. Label region 'p > Raoult prediction'. Right graph labelled 'Negative Deviation': same axes. Show dashed straight Raoult's law line. Show solid curve dipping BELOW the dashed line, labelled 'Observed p_total'. Label region 'p < Raoult prediction'. Both graphs show component contributions p_A and p_B. Dark background, orange accent labels, clean technical illustration style.
Q1.Which conditions must both be satisfied for a solution to be ideal?