The ideal gas law describes the behavior of a fictional gas.

Real gases act at room temperature as if they would shrink to nothing at absolute zero, when in fact they condense first. Before reaching absolute zero, all real gases liquefy or solidify, behavior for which the ideal gas law cannot account.

No gas obeys the conditions PV = nRT perfectly, but all gases come close at room temperatures and low pressures. This is the reason that we can apply the gas law to any gas, including an atmospheric mixture of and , without worrying about the composition of the mixture. One molecule is the same as any other in an ideal gas.

The ideal gas law assumes that attractions between molecules are negligible when compared with their energies of motion, and that the actual volumes of gas molecules are negligible in comparison with the total volume occupied by the gas.

This is close to being true at room temperature and 1 atm pressure.

At lower temperatures and slower speeds, the attractive forces between molecules no longer can be ignored.

At higher pressures, at which molecules are closer together, the volume occupied by the molecules themselves becomes an appreciable part of the volume filled by the gas. The ideal gas law begins to fail badly.

Nevertheless, under ordinary conditions the expression PV=nRT is a surprisingly good description of real gas behavior.