This paper examines the freezing and melting of water and aqueous solutions in the framework of classical nucleation theory. On the basis of thermodynamical principles, general equations for the critical germ radius and free energy are derived that express these properties as functions of temperature, solution molality, pressure, and finite size of freezing/melting particles. This theory is applied to the study of liquid-solid phase transitions: homogeneous, heterogeneous, and quasi-heterogeneous freezing of aqueous solutions and surface melting of ice. Simple analytical expressions for the corresponding freezing and melting critical temperatures are derived and solved numerically using an iteration procedure, whereby the melting and freezing temperatures are calculated for solutions with various chemical compositions, concentrations, and pressures. Comparison of the theory with experimental data shows good agreement and indicates that this approach allows reproduction of measured melting and freezing temperatures as functions of solution morality (or water saturation ratio) and pressure over wide ranges for many geophysical applications.