If I heat a rubber balloon filled with helium slowly and if the balloon is fully expandable and (the balloon) can be assumed to require no energy in its expansion,what type of thermodynamic process is taking place? Is it isobaric? Since the balloon expands the pressure exerted by the gas on balloon should increase. Then how can it be isobaric? It can't be isochoric nor isothermal. It is either adiabatic or isobaric.