Perfect gases are assumed to be comprised of infinitesimally small particles and to interact only at the point of collision. At intermediate pressures, the volume of the gas becomes lower than the ideal gas laws would predict, and attractive forces dominate the molecular interactions

Progressively higher pressures are reached, the molecules gain their proximity to one another and repulsive forces now dominate the intermolecular interactions. At low pressure, the molecules in a real gas are small relative to the mean free path, and sufficiently far apart that they may be considered only to interact close to the point of collision, and so comply with this assumption. Because the intermolecular interactions become important for real gases at moderate and high pressures, they are non-ideal gases and they no longer conform to the ideal gas laws.

The compression factor, Z expresses this behavior, and is commonly plotted as a function of pressure. It is defined as: