In: Chemistry
Due to the fact that this salt is found as a hydrated salt (first Na2SO4.10H20 and then as Na2SO4.7H2O, so, with the increase of temperature it looses moisture (water)). When the Na2SO4.10H2O is heated the solution of this compound decreases because a solid phase and a liquid phase are formed after the initial heating.
"Sodium sulfate" is a mixture of hydrated phases in shifting equilibrium given temperature and concentration. For many salts there are solubility curves as a function of temperature that are smooth (don't have any kinks). Sodium sulfate, however, has a kink in the solubility-T curve at 30 degrees as shown below: