Final answer:
An adiabatic process occurs during a gas's free expansion when no heat transfer or work is done, and the internal energy remains constant.
Step-by-step explanation:
When there is free expansion of a gas, such as when a gas confined in a thermally insulated container expands into a vacuum after a membrane is punctured, it is referred to as an adiabatic process. This process is characterized by the fact that there is no heat transfer (Q = 0) because the system is insulated, and no work is done (W = 0) because the expansion is against a vacuum.
Since the change in internal energy (∆Eint) is zero, the internal energy of the gas before and after the expansion remains the same. Other processes such as isobaric (constant pressure) or isochoric (constant volume) do not describe this scenario as they involve different constraints on pressure and volume, respectively.