| Aspect | Ideal Gas | Real Gas |
|---|---|---|
| Intermolecular forces | Zero | Present (attractive + repulsive) |
| Molecular volume | Zero (point mass) | Finite (excluded volume = b) |
| Equation of state | PV = nRT | (P + /)(V − nb) = nRT |
| Compressibility Z | Always = 1 | Z < 1 or Z > 1 depending on conditions |
| PV vs P graph | Horizontal line (PV = constant) | Dips or rises from horizontal |
| Condition for deviation | — | High P, low T |
| Condition for ideal behavior | Always (theoretical) | Low P, high T |
| Can be liquefied? | No (theoretical) | Yes, below T_c |
| Specific examples | Hypothetical | , , C, , etc. |
| KMT assumptions | Exactly satisfied | Approximately satisfied |
Special comparison — Z vs P:
- All gases: Z → 1 as P → 0 (all gases behave ideally at very low pressure)
- Most gases at moderate P: Z < 1 (attraction wins)
- All gases at very high P: Z > 1 (excluded volume wins)
- and He at all P: Z > 1 (negligible attraction from the start)