1.5.2—Real gases
- Syllabus
- First assessment 2025
- Objective
- 1.5.2
- Level
- SL
When Real Gases Deviate
Real gases deviate most from ideal behaviour at low temperature and high pressure. Low temperature reduces particle kinetic energy, while high pressure brings particles close together.
| Ideal assumption that fails | Real-gas consequence |
|---|---|
| Intermolecular forces are negligible | Attractions matter when particles have low kinetic energy and are close |
| Particle volume is negligible | Finite molecular volume matters at very high pressure |
A strong explanation names the condition, identifies the failed ideal assumption, and links it to the observed deviation.
Diagnose the cause from the condition. Cooling makes attractive forces more important because particle kinetic energy is lower; strong compression exposes both attractions and finite particle volume. Name the failed ideal assumption rather than stating only that the gas is 'non-ideal'.
Explaining Real-Gas Behaviour
Assessment in practice
1–2 marks in the selected structured examples marksHow it is assessed
Questions ask why a real-gas volume or behaviour differs from the ideal-gas prediction at high pressure or under low-temperature/high-pressure conditions.
Command terms
explain
What earns marks
Identify real-gas behaviour and link the deviation to finite molecular volume or intermolecular attractions overcoming the ideal assumptions.
Watch for
Naming high pressure or low temperature without identifying the failed ideal assumption and its particle-level consequence.
Representative question
Question 1
[Maximum number: 2]
Outline why the volume occupied by propane(g) at very high pressure is higher than the value calculated using PV=nRT.
not behaving as an ideal gas «at very high pressure»
ideal gas molecules have no volume
OR
volume of «propane» molecules is not negligible
Marking guidance:
Accept propane is a real gas for M1.
Ideal Gases Summary
Retrieve the model: ideal particles have negligible volume and forces with elastic collisions; low temperature and high pressure expose real-gas limits; molar volume and PV=nRT then connect amount, pressure, volume, and temperature.
Before calculating, check whether the question uses STP molar volume or PV=nRT, identify the fixed conditions, convert temperature to kelvin, and align pressure and volume units.