Question

Why is Gibbs free energy 0 at phase change?

Answer 1

The Gibbs free energy is not necessarily zero at a phase change.

However, since the Gibbs function depends naturally on the Thermodynamic variables `p` and `T` and since common phase transitions occurs at a constant `p` and `T`, thus, the molar/specific Gibbs function tends to be a constant during the phase change.

Thus, change in the Gibbs function is zero, i.e. `dG = 0` even though the molar/specific Gibbs function is non zero.