Editing Crystallization (section)

====Application====
Most [[chemical compound]]s, dissolved in most solvents, show the so-called ''direct'' solubility that is, the solubility threshold increases with temperature.

[[Image:Solubilita Na2SO4.png|upright=1.3|thumb|Solubility of the system Na<sub>2</sub>SO<sub>4</sub> – H<sub>2</sub>O]]

So, whenever the conditions are favorable, crystal formation results from simply cooling the solution. Here ''cooling'' is a relative term: [[austenite]] crystals in a steel form well above 1000&nbsp;°C. An example of this crystallization process is the production of [[Glauber's salt]], a crystalline form of [[sodium sulfate]]. In the diagram, where equilibrium temperature is on the [[Cartesian coordinates|x-axis]] and equilibrium concentration (as mass percent of solute in saturated solution) in [[Cartesian coordinates|y-axis]], it is clear that sulfate solubility quickly decreases below 32.5&nbsp;°C. Assuming a saturated solution at 30&nbsp;°C, by cooling it to 0&nbsp;°C (note that this is possible thanks to the [[freezing-point depression]]), the precipitation of a mass of sulfate occurs corresponding to the change in solubility from 29% (equilibrium value at 30&nbsp;°C) to approximately 4.5% (at 0&nbsp;°C) – actually a larger crystal mass is precipitated, since sulfate entrains [[Mineral hydration|hydration]] water, and this has the side effect of increasing the final concentration.

There are limitations in the use of cooling crystallization:
* Many solutes precipitate in hydrate form at low temperatures: in the previous example this is acceptable, and even useful, but it may be detrimental when, for example, the mass of water of hydration to reach a stable hydrate crystallization form is more than the available water: a single block of hydrate solute will be formed – this occurs in the case of [[calcium chloride]]);
* Maximum supersaturation will take place in the coldest points. These may be the heat exchanger tubes which are sensitive to scaling, and [[heat transfer|heat exchange]] may be greatly reduced or discontinued;
* A decrease in temperature usually implies an increase of the [[viscosity]] of a solution. Too high a viscosity may give hydraulic problems, and the [[laminar flow]] thus created may affect the crystallization dynamics.
* It is not applicable to compounds having ''reverse'' solubility, a term to indicate that solubility increases with temperature decrease (an example occurs with sodium sulfate where solubility is reversed above 32.5&nbsp;°C).