In an adiabatic process, there is no heat exchange, so Q=0. In free expansion, the gas expands into a vacuum, meaning there is no external pressure doing work, so W=0. From the first law of thermodynamics, we have: ∆U=Q−W Substituting the values, we get: ∆U=0−0=0 For an ideal gas, the internal energy U depends only on the temperature. Since ∆U=0, the temperature does not change during the process. This characteristic is described as an isothermal process. Thus, the adiabatic free expansion of an ideal gas is isothermal, which corresponds to: