In phase transitions of materials, for a melting process, the crystal solid holds a specific crystalline symmetry, while its liquid phase is spherically symmetric. Therefore, there is no symmetry relationship between the solid and liquid, and thus no continuous transition from the solid to liquid. In equilibrium cases, we have either a solid or a liquid, or a mixture of the two phases, but there is no state at which the phases of the solid and liquid are symmetry-indistinguishable. This type of transitions is called first-order phase transitions.

The first order phase transition is achieved by releasing heat (latent heat). In this case, almost all the thermodynamic quantities such as internal energy, entropy, enthalpy, volume, are discontinuous.