1. The problem statement, all variables and given/known data Suppose 2 kg of H2O at 200 C and 300 kPa are contained in a weighted piston cylinder assembly. As a result of heating at constant pressure, the temperature of the H2O increases to 400 C. Determine the change in volume, the change in internal energy, and the change in enthalpy of the H2O for this process. 2. Relevant equations By consulting water table, at 200 C the pressure of H2O is 1.5536 MPa. 3. The attempt at a solution This is a textbook example, so the answer is already known. However, my question is in regards to the concept. I have attached a figure, and everything in it is correct EXCEPT the brown line with a arrow in the left side (this is my own work). Everything else is copied direct from the book. The blue arrow is the actual process of this problem. By consulting the table, 300 kPa is obviously smaller than the saturation pressure of 1.5 MPa, and since we know Pressure is constant, this process must proceed at a pressure lower than P saturation. My question is why can't this process occur in the compressed liquid region (brown arrow)? Why the superheated region? Obviously since this is H2O, I know that's physically not possible at 200 C, but what if the substance were to be changed into something I don't know about? For example if a unknown chemical substance was used instead, and the only thing I can go by to determine its phase is the pressure difference, how do I determine whether it is in the super-heated vapor region or compressed liquid region?