I know that phase changes, such as the melting of ice, occur at constant temperature. Doesn't this imply that the change in internal energy during the phase change is zero, since the temperature remains the same? I thought that in all isothermal processes this was the case, so that the First Law of Thermodynamics becomes: dU = dQ + dW 0 = dQ + dW dQ = -dW However, according to my textbook solutions, dU is not zero. Am I misunderstanding something? Thanks in advance!