Q. Isothermal process is not possible in practice but theoretically it is. How?
Ans. In isothermal process, temperature of the system is constant and also equal to that of surroundings in contact with the system. Heat supplied to a system is possible if there is some temperature difference between the system and surrounding. Therefore, during an isothermal expansion, it could be considered that the surroundings are at a slightly elevated temperature above the system. So the necessary condition that the system shall receive heat would be met. Since the temperature difference would be small, the rate of heat transfer would be very slow. So from a practical point of view, the isothermal process is very slow. Actually this is almost impossible as a practical process. Hence, isothermal process is not possible practically.