Adiabatic free expansion

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Adiabatic free expansion is an irreversible process in which a gas expands without constraint, and during which no heat is exchanged. An example of the process is the release of a gas into a vacuum.

Real gases experience a temperature change (see Joule-Thomson effect) during free expansion. For an ideal gas, the temperature doesn't change, and the conditions before and after adiabatic free expansion satisfy p_iV_i = p_fV_f, where p is the pressure, V is the volume, and i and f refer to the initial and final states.

During free expansion, zero pressure is applied to the gas. The gas goes through states of no thermodynamic equilibrium before reaching its final state, which implies that one cannot define thermodynamic parameters as values of the gas as a whole. For example, the pressure changes locally from point to point, and the volume occupied by the gas (which is formed of particles) is not a well defined quantity.