In order to better explain my question let me give a precise situation and then state my question Say I have a well insulated rigid container containing some mass m of a saturated liquid-vapor mixture of water at some pressure P1. Initially it's at some quality x1. An electric resistance heater placed in the tank is now turned on and kept on until all the liquid in the tank is vaporized. To find the entropy change of this process I'm told to 1) Look up the values for water in a thermodynamic steam table for H2O and use the given (observed) property values (for the given property at the given pressure P1) of specific entropy from s1=(1-x1)s1,f + x1*s1,g 2) Assume the specific entropy of the water after it's done heating and further pressurizing to its fully vaporized state to be s1,g=s2 3) Multiply the mass by the change in specific entropy m(s2 - s1) thus obtaining total entropy change of the process. Now that I have established where this curiosity originated from let me ask my question During step 2 I assumed (without knowing why) that it is simply O.K. to treat s2 as s1,g this seems highly odd to me since I'm heating a rigid, insulated, container increasing the content's temperature as well as its pressure. By the time the water reaches state two it is no longer the same information on the table for sg at some new pressure and temperature P2 and T2 respectively.. Won't using the entropy values for state 2 from state 1 in the table result in inaccurate results since specific entropy should be different for a higher pressure and temperature?