Ideal solutions obey Raoult's law; deviations arise from unlike intermolecular interactions.
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An ideal solution is one in which mixing involves no change in enthalpy or volume, because the interactions between unlike molecules are the same as those between like molecules. Ideal solutions obey Raoult's law at all compositions. Real solutions show positive deviations from Raoult's law when the interactions between unlike molecules are weaker than those between like molecules, which raises the vapour pressure. Negative deviations occur when unlike interactions are stronger, which lowers the vapour pressure and often produces a volume contraction on mixing. Ethanol and water show positive deviation, while acetone and chloroform show negative deviation.
Key ideas: Ideal mixing has delta H = 0 and delta V = 0, and entropy of mixing is always positive. Positive deviation means higher vapour pressure, weaker bonds on mixing, endothermic, volume expansion. Negative deviation means lower vapour pressure, exothermic and contraction. Benzene with toluene is nearly ideal because molecules are similar.
Common mistakes and connections: Students think positive deviation means more solubility; instead it signals a tendency to separate. Another error is calling any mixture that dissolves ideal. Also, a negative deviation from Raoult's law is linked to hydrogen bond formation between unlike molecules, as in acetone and chloroform. Links to azeotropes, enthalpy of mixing and Raoult's law.
Positive deviation arises when unlike attractions are weaker (ethanol-hexane); negative deviation arises when stronger (acetone-chloroform). The interactive lesson shows this as a live animation or chart.
Ethanol-water mixtures show a positive deviation from Raoult's law, while acetone-chloroform mixtures show a negative deviation.
Ideal solutions obey Raoult's law; deviations arise from unlike intermolecular interactions.
Ideal and Non-ideal Solutions is a advanced topic in States of Matter & Solutions. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in States of Matter & Solutions first: Henry's Law, Temperature and Solubility, Raoult's Law.
The next topics in States of Matter & Solutions are Azeotropes and Colligative Properties.
Yes. The interactive lesson includes a self-check question on Ideal and Non-ideal Solutions, and the States of Matter & Solutions quiz draws on every topic in this area.
Ideal and Non-ideal Solutions is topic 85 of 109 in States of Matter & Solutions.