The enthalpies of phase change have been studied by means of CDR-34p Differential Scanning Calorimetry(DSC) and Phase diagram for Na₂SO₄-H₂O system. The current study found that latent heat for inorgonic hydrated salt is made of reaction enthalpies, dissolution heats, integral dissolution heats. In addicition, the latent heat of phase change was calculated. Compared with experimental data, the relative error of the calculated results is just 2%.