Why do real gases depart from ideal behavior at high density?

Real gases depart from ideal behavior at high density primarily because the assumptions of the ideal gas model—which works well at low pressures and high temperatures—no longer hold true.12

At high densities, two key factors cause these deviations:

• Intermolecular Interactions: Unlike the ideal model, which assumes molecules move independently, real gases are subject to intermolecular forces. These attractive and repulsive forces significantly influence molecular behavior when molecules are packed closely together.34
• Molecular Volume: The ideal gas law assumes that the volume of the molecules themselves is negligible compared to the total volume of the gas. At high densities, the actual volume occupied by the molecules becomes a non-negligible fraction of the total space, contradicting this assumption.12

Furthermore, at high densities, a description based solely on two-body collisions becomes inadequate, as collisions involving three or more molecules simultaneously become significant. These complexities are often addressed using the van der Waals equation, which introduces constants to account for molecular cohesion and the finite volume of gas particles.567