Editing Wilhelm Ostwald (section)

=== Crystallization ===
Ostwald studied the [[crystallization]] behavior of solids, especially those solids that are capable of crystallizing in different forms, in the phenomenon known as [[Polymorphism (materials science)|polymorphism]]. He discovered that solids do not necessarily crystallize in their most thermodynamically stable form but instead sometimes crystallize preferentially in other forms dependent on the relative rates of crystallization of each polymorphic form. Ostwald found that the relative rates were dependent on the surface tension between the solid polymorph and the liquid form. Many common materials exhibit this type of behavior, including [[minerals]] and various [[organic compounds]]. This finding came to be known as [[Ostwald's rule]].<ref name="Ostwald Rule">{{cite thesis |last1=Wang |first1=Tingting |title=Breakdown of the Ostwald step rule – The precipitation of calcite and dolomite from seawater at 25 and 40 °C |date=2013 |url=https://escholarship.mcgill.ca/concern/theses/sn00b224t }}</ref>

Ostwald realized that solid or liquid solutions can continue to evolve over time. While the a non-thermodynamically preferred polymorph may crystallize first, more thermodynamically stable forms can continue to develop as the solution ages. Often this results in large crystals forming, since they are more thermodynamically stable than are large numbers of small crystals. This phenomenon came to be known as Ostwald Ripening and is observed in many situations. An everyday example is the gritty texture that ice cream develops as it ages. On a [[geologic timescale]], many [[minerals]] exhibit Ostwald Ripening as their crystal forms evolve as the mineral ages.<ref name="Clay Minerals">{{cite journal |last1=Jahren |first1=J.S. |title=Evidence of Ostwald Ripening Related Recrystallization of Diagenetic Chlorites From Reservoir Rocks Offshore Norway |journal=Clay Minerals |date=1991 |volume=26 |issue=2 |page=169 |doi=10.1180/claymin.1991.026.2.02 |bibcode=1991ClMin..26..169J |citeseerx=10.1.1.604.4580 |s2cid=97430142 }}</ref>

Related to solubility and crystallization was Ostwald's finding that dissolution of a solid depends on the size of the crystal. When the crystals are small, typically less than a [[micron]], the solubility of the solid in the solution phase is increased. Ostwald quantified this effect mathematically in a relationship that became known as the [[Ostwald-Freundlich equation]]. Ostwald first published his finding in 1900, and his mathematical equation was refined by German chemist [[Herbert Freundlich]] in 1909. This mathematical relationship also applies to the partial pressure of substance in the system. The Ostwald-Freundlich equation takes into account the surface tension of the particle in the system, in addition to curvature and temperature. The size dependence of solubility is sometimes utilized in the formulation of [[pharmaceuticals]] that have low solubility so as to enhance their uptake by the patient. The size dependence also has a role in Ostwald Ripening.<ref name="Freundlich eqn">{{cite journal |last1=Eslami |first1=Fatemeh |last2=Elliott |first2=Janet A. W. |title=Role of Precipitating Solute Curvature on Microdrops and Nanodrops during Concentrating Processes: The Nonideal Ostwald–Freundlich Equation |journal=Journal of Physical Chemistry B |date=2014 |volume=118 |issue=50 |pages=14675–86 |doi=10.1021/jp5063786 |pmid=25399753 |doi-access=free }}</ref>

[[File:Liesegang rings Saginaw Hill AZ.jpeg|left|thumb|Liesegang rings at Saginaw Hill, Arizona, USA]]
Collaborating with German chemist [[Raphael E. Liesegang]], Ostwald recognized that substances can crystallize in a periodic fashion wherein the crystallization behavior follows a spatial or temporal pattern. In certain circumstances, the result of this periodic crystallization behavior is easily visually observed, for example, in various [[geologic formations]]. Liesegang had previously investigated this phenomenon in specific laboratory experiments, showing his results to Ostwald. Ostwald then developed a mathematical model for the phenomenon that served to explain the observations and realized how widespread is the periodic crystallization behavior. These observations came to be known as [[Liesegang rings]].<ref name="Liesegang">{{cite web |title=A Short History of "Liesegang Rings" |url=https://www.insilico.hu/liesegang/history/history.html |website=insilico.hu |publisher=In Silico, Ltd. |access-date=7 August 2020}}</ref>