Real gases deviate from ideal gas behavior because their particles have finite volume and experience intermolecular forces. These effects become most significant at high pressure and low temperature. This is required at both SL and HL in Structure 1.5.
The Reasoning
The ideal gas model makes two important assumptions: gas particles have negligible volume, and there are no intermolecular forces between them. Under many conditions, these approximations work well, so an ideal gas obeys .
Real gas particles do occupy space. At high pressure, particles are forced close together, so their own volume becomes significant compared with the volume of the container. The volume available for particle movement is therefore smaller than the measured container volume.
Real particles also attract one another through intermolecular forces. At low temperature, particles have lower average kinetic energy and move more slowly. Attractive forces can then influence their motion more strongly, making particles collide with the container walls less frequently or less forcefully than the ideal model predicts. Consequently, the measured pressure may be lower than the ideal pressure.
| Condition | Why deviation increases |
|---|---|
| High pressure | Particles are close together, so particle volume and intermolecular forces are no longer negligible. |
| Low temperature | Lower kinetic energy makes intermolecular attractions more significant relative to particle motion. |
| Low pressure and high temperature | Particles are far apart and move rapidly, so the gas behaves more ideally. |
A common misconception is that gases become non-ideal because particles stop moving randomly. They still move randomly; the deviation occurs because the assumptions of negligible particle volume and zero intermolecular forces become inaccurate.
Exam Technique
For an IB explain question, identify both failed assumptions and connect each to the relevant condition: finite particle volume matters at high pressure, while intermolecular attractions matter especially at low temperature. Do not merely state that real gases deviate; explain the particle-level mechanism.
