Ideal gas, a theoretical model of a gas in which the interaction of the gas particles is not taken into account (the mean kinetic energy of the particles is much greater than the energy of their interaction). A distinction is drawn between the classical and the quantum ideal gas. The properties of a classical ideal gas are described by the laws of classical physics — the Clapeyron equation and its particular cases: the Boyle — Mariotte law and Gay-Lussac's law. The particles of a classical ideal gas are distributed in energy according to the Boltzmann distribution (see Boltzmann statistics). Real gases are described well by the classical ideal-gas model if they are sufficiently rarefied. As the temperature of a gas falls or its density rises, the wave (quantum) properties of the particles of an ideal gas may become significant, if their de Broglie wavelengths at velocities of the order of thermal velocities become comparable with the distances between the particles. The behaviour of a quantum ideal gas made up of particles with integer spin is then described by Bose — Einstein statistics, while the behaviour of a gas of particles with half-integer spin is described by Fermi — Dirac statistics (see Quantum statistics).