Hi all, Will like to check if my below thoughts are right: 1) For a totally reversible process, it has to be both internally and externally reversible. And the definition of externally reversible means heat transfer between a system and reservoir where the temperature are the same. 2) For an isentropic process, it has to be internally reversible and adiabatic. From the above 2 definition, can i say that A) For a totally reversible process, it will not be isentropic since it involve heat transfer, albeit the temperature difference is infinitisimal small.(based on definition of externally reversible) B) An isentropic process is not a totally reversible process but only an internally reversible process For another question on the same topic, in an adiabatic reversible expansion of a piston system( i.e isentropic) , can I visualise the reason this process is seen as isentropic is because although the volume increases, meaning the molecules are more random(increase entropy), the counter effect is brought about by the reduction in pressure and temperature, hence slowing down the molecule speed, resulting in a zero entropy change??