Despite intense experimental and theoretical efforts, it is still unclear why at temperatures of phase transitions, materials change completely their physical properties. Experimentally we are able to measure any microscopic, mesoscopic, and macroscopic variations of physical observables, but we still do not have a universal theoretical model that can predict all phase transitions, qualitatively and quantitatively. For instance, experimentally we can measure all physical properties of superconductors, but we do not have a unique theoretical model that is able to describe both types of superconductivity, namely the conventional and unconventional superconductivity. Here I present a simple quantum-mechanical and universal theoretical model that is able to calculate and predict critical temperatures for phase transitions: between magnetic order and disorder, superconductivity and normal conductivity, superconductivity and insulator, metal and insulator, melting and crystallization and many other phase transitions. All these phase transitions are described with four wave functions that are eigenfunctions of the Hamiltonian and the momentum operators.