The solubility of most materials increases with temperature. However, a number of examples exist, where the solubility shows the reverse trend (sodium sulfate, calcium carbonate, iron sulfate dihydrate, to name but a few). The dependence of the solubility upon temperature is best represented graphically in the form of a solubility curve, which maps the composition of a solution at the solubility limit onto the temperature. A generalized solubility curve is shown in Figure 1 below. The solubility is a continuous function of temperature for a given form of a given material. Polymorphs of the same material, solvates and hydrates (vide infra)generally have different solubilities. A change in solid form is normally evidenced by a discontinuity in the solubility curve, which results from different slope, that is, temperature dependence, of the solubility of the respective forms. In Figure the solubility of a fictitious substance A that forms three phases, two hydrates and one anhydrous phase, is shown. Two discontinuities can be seen marking the transition points between the stable regions for the trihydrate and monohydra (left) and the monohydrate and anhydrous phase (right). The solubilities of the respective stable phases are indicated by the solid line. In addition, the solubilities of the different metastable forms are indicated by the dotted line.
index terms: Industrial Crystallization, Solubility, Phase Diagrams, Nucleation, Crystal Growth, Polymorphism, Hydrates and Solvates, Crystal Habit, Crystallization
Technology, Melt Crystallization.
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