What change of state is liquid to solid?
Liquid to solid phase transitions are classified under first order phase transitions.
Ehrenfest classified phase transitions among first or second order depending on the order of derivatives of the Gibb's function that undergoes discontinuous changes at the phase boundary.
In the solid-liquid phase transition, there is a discontinuous change of molar entropy and molar volume of the system which are first derivatives of the molar Gibbs' function hence classifying it as a first order phase transition.
It may be noted that first order phase transitions have latent heat associated with them.
As you may be familiar there is a latent heat associated with the melting of ice or freezing of water. Therefore, fusion (or it's reverse process freezing) are first order phase transitions.