The internal Energy E is this: [itex]\Delta[/itex] E = q + w = q - P[itex]\Delta[/itex]V where q is heat, w is work, and V is volume Phase changes occur when you change internal energy of the system, right? I am assuming that this means when you go from gas to liquid to solid, you must decrease the internal energy (remove heat). And when you go from a solid to liquid to a gas, you must add internal energy (add heat). That obviously makes sense, but what happens when you involve the work part of internal energy. When you do work on the system (volume decreases), you increase its internal energy, yet compression causes a gas to turn to a liquid. How can this be since you are increasing the internal energy? Or am i understanding this wrong? Is it the increase in internal energy or the increase in heat that determines phase changes?