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How crystallization can give puriest form of matter |
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Answer» Thanks The crystallization process consists of two major events,\xa0nucleation\xa0and\xa0crystal growth\xa0which are driven by thermodynamic properties as well as chemical properties. In crystallization\xa0Nucleation\xa0is the step where the solute molecules or atoms dispersed in the\xa0solvent\xa0start to gather into clusters, on the microscopic scale (elevating solute concentration in a small region), that become stable under the current operating conditions. These stable clusters constitute the nuclei. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by many different factors (temperature,\xa0supersaturation, etc.). It is at the stage of nucleation that the atoms or molecules arrange in a defined and\xa0periodic\xa0manner that defines the\xa0crystal structure\xa0— note that "crystal structure" is a special term that refers to the relative arrangement of the atoms or molecules, not the macroscopic properties of the crystal (size and shape), although those are a result of the internal crystal structure.The\xa0crystal growth\xa0is the subsequent size increase of the nuclei that succeed in achieving the critical cluster size. Crystal growth is a dynamic process occurring in equilibrium where solute molecules or atoms precipitate out of solution, and dissolve back into solution. Supersaturation is one of the driving forces of crystallization, as the solubility of a species is an equilibrium process quantified by Ksp. Depending upon the conditions, either nucleation or growth may be predominant over the other, dictating crystal size.Many compounds have the ability to crystallize with some having different crystal structures, a phenomenon called\xa0polymorphism. Certain polymorphs may be\xa0metastable, meaning that although it is not in thermodynamic equilibrium, it is kinetically stable and requires some input of energy to initiate a transformation to the equilibrium phase. Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of the same compound exhibit different physical properties, such as dissolution rate, shape (angles between facets and facet growth rates), melting point, etc. For this reason, polymorphism is of major importance in industrial manufacture of crystalline products. Additionally, crystal phases can sometimes be interconverted by varying factors such as temperature, such as in the transformation of\xa0anatase\xa0to\xa0rutile\xa0phases of\xa0titanium dioxide. |
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