Why must solid-liquid transitions always be separated by a first-order transition line on the phase diagram?

Answer

They involve a change in symmetry (breaking lattice translation symmetry).

Solid-liquid transitions always involve a change in symmetry, specifically breaking lattice translation symmetry, which forces them to be separated by a first-order transition line that continues indefinitely.

Why must solid-liquid transitions always be separated by a first-order transition line on the phase diagram?

#Videos

Phase Transitions & Bifurcations - YouTube

Thermodynamicscausematterphase transition