---
title: "Ideal and Real Solutions: Raoult, Henry and Excess Functions"
description: "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 wha"
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source: https://lightmysky.com/learn/science/ideal-and-real-solutions-raoult-henry-and-excess-functions-mt_DRHoU0yg3Z.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Chemistry · Ages 20 to 22
Page: https://lightmysky.com/learn/science/ideal-and-real-solutions-raoult-henry-and-excess-functions-mt_DRHoU0yg3Z

## Ready when they can

- 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

## Lesson: When solutions stop being ideal

A solution is ideal when every molecule feels the same neighbours it felt in its pure liquid. Use the Raoult law for the solvent and the Henry law for the dilute solute, since each law fits one end of the range. Henry constants also link a dissolved gas to its partial pressure, which is why fizzy drinks hiss on opening.

**Example.** 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. Stronger unlike contacts hold them in and lower it.

Ethanol and water sit above the ideal line, a positive deviation, because mixing disrupts the hydrogen bond networks each pure liquid enjoyed. 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.

**Tip.** 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.

**Recap.** Raoult fits the solvent, Henry fits the solute, and any gap from ideal tells you about the contacts.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Phase Diagrams, the Clapeyron Equation and the Phase Rule](https://lightmysky.com/learn/science/phase-diagrams-the-clapeyron-equation-and-the-phase-rule-mt_U-zj6r4WFM)

## Opens up

- [Putting the Solvent Into a Calculation](https://lightmysky.com/learn/science/putting-the-solvent-into-a-calculation-mt_3SCFfcvoei)
- [Electrolyte Solutions and the Debye-Huckel Limiting Law](https://lightmysky.com/learn/science/electrolyte-solutions-and-the-debye-huckel-limiting-law-mt_6wAcvWy0dv)