🟣 This is Higher Level (HL) content.
No real gas behaves ideally under all conditions. Deviations occur overwhelmingly under extreme conditions: specifically, low temperature and high pressure.
Why Real Gases Deviate
❄️ Low Temperature
At low temperatures, particle kinetic energy decreases significantly. Particles move sluggishly, giving intermolecular forces (London/dipole) sufficient time to take effect.
These attractive forces pull particles closer together, so they strike the walls with less force and less frequency.
Result: Observed pressure is lower than the ideal model predicts.
🔴 High Pressure
Under immense pressure, particles are forced into close proximity. The postulate that particles have "negligible volume" collapses. Their electron clouds occupy a significant fraction of the space.
There is a hard physical limit to compression: particles cannot be compressed to nothing.
Result: Observed volume is greater than the ideal model predicts.
🔑 When Does a Gas Behave Most Ideally?
A gas behaves most like an ideal gas at high temperature (particles too fast for IMFs to act) and low pressure (particles too far apart for volume to matter). Noble gases and hydrogen approximate ideal behaviour best because their intermolecular forces are the weakest.
Compressibility Factor (Z) vs Pressure
Summary: Which Postulate Fails?
| Condition | Postulate That Fails | Observed Effect |
|---|---|---|
| Low temperature | "No intermolecular forces" | Preal < Pideal |
| High pressure | "Negligible particle volume" | Vreal > Videal |
Which Gases Deviate the Most?
Gas Type and Deviation from Ideality
- Most ideal: Small, non-polar molecules with weak London forces, such as He, H₂, and Ne. These have the weakest intermolecular forces and smallest particle volume.
- Least ideal: Large, polar molecules with strong intermolecular forces, such as NH₃, H₂O, and HCl. Their significant attractions cause the greatest deviations at low T and high P.