An ideal gas is characterized by the fact that the size of its particles can be neglected compared to the distances between them, and that interactions between the particles can be ignored. In particular, hydrogen and the noble gases from the last main group of the periodic table can be considered nearly ideal gases due to their chemical inertness.
The state of an ideal gas is completely defined by its temperature, volume, and pressure. In the following, unless otherwise noted, only ideal gases will be considered and examined.