Closed systems are those in which mass remains constant but energy can flow through the system's boundaries, which is why they're also known as control mass systems.Example: A closed system is a gas-filled container with a movable piston since altering the volume by adjusting the piston causes a change in the temperature of the gas, indicating that thermal energy flowed in the system but the gas's mass remained constant.Open systems are those in which mass and energy can both flow across the system's boundaries. The control volume is another name for them.Example: A water heater is an open system since cold water enters the system while hot water exits, implying that energy and mass both move over the system's boundaries (control volume).