text: According to Hopkins and Wulff (1965), the decrease of calcium hydroxide solubility with temperature was known since the works of Marcellin Berthelot (1875) and Julius Thomsen (1883) (see Thomsen–Berthelot principle), when the presence of ions in aqueous solutions was still questioned. Since, it has been studied in detail by many authors, a.o., Miller and Witt (1929) or Johnston and Grove (1931) and refined many times (e.g., Greenberg and Copeland (1960); Hopkins and Wulff (1965); Seewald and Seyfried (1991); Duchesne and Reardon (1995)).
  The reason for this rather uncommon behavior is that the dissolution of calcium hydroxide in water involves an entropy decrease, due to the ordering of water molecules around the doubly charged calcium ion, This counter-intuitive temperature dependence of the solubility is referred to as "retrograde" or "inverse" solubility. The variably hydrated phases of calcium sulfate (gypsum, bassanite and anhydrite) also exhibit a slight retrograde solubility due to the presence of the calcium ion. However, other calcium salts like calcium chloride show an increase in solubility since the enthalpy change is larger and dominates the free energy change during dissolution. In all cases, the dissolution is exothermic.