A thermodynamic process that occurs at constant volume is called an isochoric process. Consider evaporation occurring inside a sealed ecosystem. Since the water vapour does not escape the container, the volume remains constant, and the process is isochoric. At constant volume, no work is done by the system. Now, applying the first law of thermodynamics, the heat added to the system equals the change in internal energy. In a pV diagram, this process is represented by a vertical line parallel to the pressure axis. A thermodynamic process occurring at constant pressure is an isobaric process. When water evaporates from a river, the water vapor escapes. However, the atmospheric pressure on the system remains constant, and hence this process is isobaric. Here, the work done equals the product of pressure and volume change. Therefore, the heat added to the system is utilized to increase the system's internal energy and perform the work. The pV diagram for an isobaric process is a horizontal line parallel to the volume axis.