When one says "internal energy is dependent only on temperature", at an introductory level it primarily means that internal energy is not dependent on pressure.

In higher-level thermodynamics, one finds out that the internal energy can be defined as being dependent on a variety of things, depending on how one sets up one's system.

But that's getting ahead of things.

The internal energy absolutely does change when a substance changes phase.

The reason why the water molecules "lock into place" to form ice is because they've had enough energy removed from them that they can't "get loose" from the ice crystal structure.

yes sir iam somewhat able to understand it,water molecules in ice are more closely packed because they have lost some internal energy right? one thing that is clear is,this is somewhat high level,so i prefer remembering it as a fact "in phase change internal energy of the molecule decreases even if the temperature is constant"

if it is not a phase change,then will the internal energy be dependent only on temperature atleast in high school themrodynamics?

@AjaySabarish "water molecules in ice are more closely packed because they have lost some internal energy right?"

Them being more closely packed is part of what happens

But the water molecules also are unable to move around relative to one another once that energy has been removed

@AjaySabarish "if it is not a phase change,then will the internal energy be dependent only on temperature atleast in high school themrodynamics?"

Yes, I believe you can count on this

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