Two ways of cooling a (real) gas are exchanging heat or doing external work. Let's assume we have a fixed volume of gas that does no work nor exchanges any heat (with surroundings i.e. adiabatic). Is it possible for such a gas parcel to cool at all? Or heat up? My intuition says no. An isolated, adiabatic, isochoric parcel of gas cannot change T. But what prevents it fundamentally? First law constraints would say U = const. (since W & Q both zero). But for a real gas U=fn(T,P) right? So theoritically T could change & P changes to compensate & keep U a constant? So can it? Or can't it? What gives? Is it a second law constraint?