Crystallizing” and “Crystallized” redirect here. For the types of crystallizers pdf, see Crystalised. Some of the ways by which crystals form are precipitating from a solution, melting, or more rarely deposition directly from a gas.
Attributes of the resulting crystal depend largely on factors such as temperature, air pressure, and in the case of liquid crystals, time of fluid evaporation. Crystallization occurs in two major steps.
The first is nucleation, the appearance of a crystalline phase from either a supercooled liquid or a supersaturated solvent. The second step is known as crystal growth, which is the increase in the size of particles and leads to a crystal state.
An important feature of this step is that loose particles form layers at the crystal’s surface lodge themselves into open inconsistencies such as pores, cracks, etc. The majority of minerals and organic molecules crystallize easily, and the resulting crystals are generally of good quality, i. However, larger biochemical particles, like proteins, are often difficult to crystallize. In chemical engineering, crystallization occurs in a crystallizer.
Crystallization is therefore related to precipitation, although the result is not amorphous or disordered, but a crystal. Time-lapse of growth of a citric acid crystal.
The video covers an area of 2. 5 mm and was captured over 7. The crystallization process consists of two major events, nucleation and crystal growth which are driven by thermodynamic properties as well as chemical properties. These stable clusters constitute the nuclei.
Therefore, the clusters need to reach a critical size in order to become stable nuclei. The crystal growth is 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 polymorphism.
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.
Snowflakes are a very well-known example, where subtle differences in crystal growth conditions result in different geometries. There are many examples of natural process that involve crystallization.
Crystal formation can be divided into two types, where the first type of crystals are composed of a cation and anion, also known as a salt, such as sodium acetate. The second type of crystals are composed of uncharged species, for example menthol. The formation of a supersaturated solution does not guarantee crystal formation, and often a seed crystal or scratching the glass is required to form nucleation sites.