B.3.8—Ideal gas approximation limits
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- HL
Check When the Ideal-Gas Approximation Holds
Good approximation
A real gas is closest to the ideal-gas model at relatively low pressure and low density, where particles are far apart and intermolecular forces and particle volume are small compared with the container volume.
Temperature matters
Higher temperature gives particles more kinetic energy, making attractive interactions less important. Low temperature increases the importance of intermolecular attractions and can bring the gas closer to condensation.
Where it fails
At high pressure or high density, particles are crowded: their finite size and interactions matter. Near phase changes, the ideal model is especially unreliable.
Use the full condition
Do not state only “high temperature”. The reliable region is generally high temperature together with low pressure or low density.
B.3.8 Exam Analysis
Assessment in practice
1–2 marksHow it is assessed
The evidence asks why an ideal-gas pressure prediction becomes unreliable at very small volume/high pressure and asks for the pressure/density conditions where the model is valid.
Command terms
State / Suggest
What earns marks
State that the ideal approximation is better at low pressure and low density, and generally higher temperature. At very small volumes or high pressures, particle volume and intermolecular forces violate model assumptions.
Watch for
Saying only high temperature without low pressure/low density, or claiming the model improves at high pressure.
Representative question
Question 1
[Maximum number: 1]
Under which conditions of pressure and density will a real gas approximate to an ideal gas?
Pressure
Density
high
high
high
low
low
high
low
low
D