**Ideal gas** (or *perfect gas*) is a hypothetical gas which obeys gas laws exactly. An ideal gas consists of molecules which occupy negligible space and among which negligible forcess exist (except at collisions). All collisions between molecules or between molecules and the walls of container are perfectly elastic, as the molecules have no means of storing energy except as translational kinetic energy. Ideal gas is a good approximation for diluted real gases.

At lower temperatures and higher densities, it may be necessary to treat effects of quantum-mechanical statistics even before those of interactions. In such cases, the concept of an ideal gas is extended to that of an ideal Fermi gas or Bose gas.

The ideal gas law relates physical conditions as follows:

P V = n R T

where

- P is the pressure of the gas
- V is the volume containg the gas
- n is the molar mass
- R is the universal gas constant
- T is the temperature of the gas