Raoult’s Law is a key concept in IB Chemistry Topic 1 (States of Matter) and Topic 7 (Equilibrium). It explains how adding a solute affects the vapor pressure of a solvent, and it forms the foundation for understanding solutions, ideal behavior, boiling point elevation, and non-ideal mixtures. This article explains Raoult’s Law clearly, with the level of detail needed for IB exams.
What Is Raoult’s Law?
Raoult’s Law states that the vapor pressure of a solvent in an ideal solution equals the vapor pressure of the pure solvent multiplied by its mole fraction in the solution.
Mathematically:
P(solution) = X(solvent) × P°(solvent)
Where:
- P(solution) = vapor pressure of solvent in the mixture
- X(solvent) = mole fraction of solvent
- P°(solvent) = vapor pressure of pure solvent
This relationship shows that vapor pressure decreases when a non-volatile solute is added.
The Core Idea Behind Raoult’s Law
When a solute is added to a solvent:
- Fewer solvent molecules are at the surface
- Fewer particles can escape into the vapor phase
- Vapor pressure decreases
This is why saltwater boils at a higher temperature and freezes at a lower temperature than pure water.
Raoult’s Law mathematically describes this effect for ideal solutions.
What Is an Ideal Solution?
An ideal solution is one where:
- The intermolecular forces between solute and solvent are similar to those within each pure substance
- No energy is absorbed or released when mixing
- Interactions behave as expected from simple dilution
Examples that approximate ideal behavior:
- Hexane + heptane
- Ethanol + propanol
These mixtures consist of molecules with similar sizes and intermolecular forces.
Vapor Pressure Lowering
Adding a non-volatile solute (one that does not evaporate) reduces vapor pressure.
Example:
Salt (NaCl) in water does not evaporate.
Therefore, water’s vapor pressure decreases according to Raoult’s Law.
Lower vapor pressure leads to:
- Higher boiling point
- Lower freezing point
- Changes in equilibrium behavior
This underpins many colligative property calculations.
Raoult’s Law for Volatile Solutes
If both components are volatile (e.g., two liquids), each contributes to total vapor pressure.
Then:
P(total) = X(A)P°(A) + X(B)P°(B)
This is essential for understanding:
- Fractional distillation
- Ideal mixtures
- Behavior of miscible liquids
The more volatile component contributes more to the vapor.
Positive and Negative Deviations (HL Concept)
Real solutions often deviate from Raoult’s Law.
Positive deviations
- The mixture has weaker intermolecular forces than expected
- Vapor pressure is higher than Raoult’s Law predicts
Examples: - Ethanol + hexane
- Acetone + carbon disulfide
Because molecules attract each other less strongly, they escape more easily.
Negative deviations
- The mixture has stronger intermolecular forces than expected
- Vapor pressure is lower
Examples: - Nitric acid + water
- Chloroform + acetone
Extra attraction makes vaporization harder.
These deviations are often tested in HL data-based questions.
Applications of Raoult’s Law
1. Boiling Point Elevation
Lower vapor pressure means a solution must reach a higher temperature before its vapor pressure equals external pressure.
2. Freezing Point Depression
Lower vapor pressure leads to a lower freezing point.
3. Fractional Distillation
Raoult’s Law helps predict how mixtures separate based on volatility differences.
4. Calculating vapor composition
When a mixture evaporates, the vapor is richer in the more volatile component.
Common IB Misunderstandings
- Thinking a solute increases vapor pressure (it decreases it).
- Forgetting that Raoult’s Law applies only to ideal solutions.
- Confusing mole fraction with percent composition.
- Assuming all deviations are positive—both types exist.
Carefully checking the nature of the solution avoids these errors.
FAQs
Does Raoult’s Law apply to all solutions?
No. It applies best to ideal solutions. Real solutions often show deviations due to differences in intermolecular forces.
Why does vapor pressure decrease when solute is added?
Because fewer solvent molecules reach the surface, reducing evaporation.
Can Raoult’s Law be used for solutions with volatile solutes?
Yes. Each volatile component contributes its own partial pressure based on its mole fraction.
Conclusion
Raoult’s Law describes how vapor pressure depends on the mole fraction of solvent in an ideal solution. Adding a solute lowers vapor pressure, influences boiling and freezing points, and determines how mixtures behave during distillation. Understanding this law provides essential insight into thermodynamics and solution chemistry in the IB curriculum.
