Which statement is NOT an assumption of the ideal gas model?

• A. Gas particles occupy negligible volume.
• B. Collisions are elastic.
• C. There are no intermolecular forces.
• D. Gas identity affects behavior at fixed temperature and pressure.

Answer: (D)

The idea being tested is what the ideal gas model assumes about how gases behave. In this model, gas particles are considered point-like with negligible volume, they collide elastically, and there are no intermolecular forces. Under these simplifying assumptions, all gases behave identically at the same temperature and pressure; their chemical identity doesn’t affect the macroscopic behavior described by the ideal gas law.

So the statement that gas identity affects behavior at fixed temperature and pressure is not an assumption of the model. It contradicts the core idea that, for an ideal gas, identity doesn’t matter and all gases follow the same PVT relationship.

The other statements align with the model: vanishing particle volume, elastic collisions, and no intermolecular forces. Real gases only approximate these conditions, which is why deviations occur when conditions push the system away from the ideal assumptions.