Real gases deviate most from ideal behavior at low temperature and high pressure. Under these conditions, intermolecular attractions and the finite volume of gas particles become significant, so the assumptions of the ideal gas model no longer hold.
In IB Chemistry Structure 1.5, an ideal gas is modeled as particles with negligible volume and no intermolecular forces. Its behavior is described by the ideal gas equation:
Real gas particles have volume and attract one another. These differences matter most under the following conditions:
| Condition | Why deviation increases |
|---|---|
| Low temperature | Particles have lower average kinetic energy and move more slowly. Intermolecular attractions therefore have a greater effect on their motion and may pull particles toward one another, eventually causing condensation. |
| High pressure | Particles are forced close together. Their own volume is no longer negligible compared with the container volume, and intermolecular attractions or short-range repulsions become significant. |
At high temperature, particles have enough kinetic energy for intermolecular attractions to have relatively little effect. At low pressure, particles are far apart, so both their individual volumes and intermolecular forces are negligible. Real gases therefore behave most ideally at high temperature and low pressure.
The extent of deviation also depends on the gas. Large, highly polarizable molecules and polar molecules generally experience stronger intermolecular forces than small, non-polar particles, so they tend to deviate more under comparable conditions.
A common misconception is that high pressure alone makes particles larger. It does not. High pressure reduces the distance between particles, making their existing finite volume and intermolecular forces important.
Exam technique: For an IB “explain” question, state both conditions and link each one to a failed ideal-gas assumption. Do not simply write “low temperature and high pressure”; explain lower kinetic energy, reduced particle separation, intermolecular attractions, and non-negligible particle volume.