# Why is change in internal energy not equal to zero during a phase transition? [closed]

Many books state that when there is no change in temperature $$\Delta U$$ is equal to zero. Why is this not applicable for phase transitions? I have had this question for a while now and have been unable to find an answer to it

• "when there is no change in temperature ΔU is equal to zero." This is true of ideal gases, not general at all! – Buck Thorn Sep 21 at 8:02

Such books forget to mention, or you may have not noticed such notes, that $$\mathrm{d}T=0 \implies \mathrm{d}U=0$$ does not apply to phase transitions because there is heat exchange at constant temperature, related to different energy of different phases at the same temperature.