A real gas behaves most like an ideal gas at low pressure and high temperature, particularly when it is far from condensation. This applies to both SL and HL in IB Physics topic B.3 Gas laws.
An ideal gas is a model in which particles are treated as point particles with negligible volume, and intermolecular forces are assumed to be negligible except during elastic collisions. Its macroscopic behaviour follows the ideal gas law:
The conditions determine how closely a real gas satisfies these assumptions.
| Condition | Why behaviour becomes more ideal |
|---|---|
| Low pressure | The gas has low density, so particles are widely separated. Their own volume is negligible compared with the container's volume. |
| High temperature | Particles have greater average kinetic energy, so intermolecular attractions have less influence on their motion. The gas is also less likely to condense. |
For example, if a gas occupies and its particles have a combined physical volume of only , particle volume is approximately of the container volume and can reasonably be neglected. If the gas is compressed to , particle volume is no longer negligible, so ideal behaviour fails.