Ideal and Real Solutions: Raoult, Henry and Excess Functions
A solution is ideal when every molecule feels the same neighbours it felt in the pure liquid. Two limiting laws describe the ends of that range, and the gap between measured and ideal behaviour is what excess functions record.
What a learner can do afterwards
- Applies Raoult's law to a solvent and Henry's law to a dilute solute, and says which applies where
- Reads a vapour pressure against composition plot and identifies positive or negative deviation
- Links the sign of the deviation to the relative strength of the interactions involved
- Explains why an azeotrope cannot be separated by simple distillation
1 · Read
A solution is ideal when every molecule feels the same neighbours it felt in its pure liquid. Raoult's law fits the solvent: its vapour pressure is its mole fraction times its pure vapour pressure, so at x = 0.90 with a pure value of 12.0 kPa it is 10.8 kPa. Henry's law fits the dilute solute and links a dissolved gas to its partial pressure, which is why fizzy drinks hiss on opening.
Plot vapour pressure against composition and compare with the ideal line. Points above the line mean positive deviation, points below mean negative deviation. Weaker contacts between unlike molecules push molecules out and raise the pressure, and such mixing usually takes in heat. Stronger unlike contacts hold them in and lower it.
Ethanol and water sit above the ideal line, a positive deviation, so molecules escape the mixture more easily than an ideal one. At one composition the liquid and the vapour share the same mix. That is the azeotrope, and boiling cannot shift it, so simple distillation stalls there.
Excess functions record the gap between measured and ideal behaviour. Read their sign the same way you read deviation: positive deviation pairs with positive excess Gibbs energy. Distillation curves flatten at the azeotrope, marking the limit of that separation.
Raoult fits the solvent, Henry fits the solute, and any gap from ideal tells you about the contacts.
The rewritten readings were written for this dataset. Reading summaries adapted from Beginning Chemistry (Ball), Chemistry LibreTexts (CC BY-NC-SA 3.0), chem.libretexts.org.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.