The Process of Crystallization
Crystallization refers to the process of crystal formation from a solution, a molten substance, or a gas. The entire process of crystallization can be divided into two main stages - nucleation and crystal growth.
Nucleation refers to the accumulation of the solute into clusters. However, the clusters should be stable enough to ensure the formation of crystals. Otherwise, they can dissolve again in the solution.
The stable clusters form nuclei, and for this, the clusters have to attain a critical size determined by the operating conditions, like temperature and supersaturation (refers to a solution containing more of the dissolved material than what can be dissolved under normal conditions).
At this stage of nucleation, atoms get arranged in geometrical shapes in a periodic or repeated manner, which determines the structure of the crystal.
The next stage of crystallization is crystal growth, which refers to the growth of the stable nuclei. This helps the crystals attain the critical cluster size, after which it can no longer dissolve in the solution. Nucleation and crystal growth take place simultaneously as long as supersaturation exists.
So, the most important condition for crystallization is the existence of supersaturation, as it determines the rate of nucleation and crystal growth. When supersaturation ceases to exist, the solid-liquid system attains equilibrium and the process of crystallization comes to an end.
Crystals can be formed due to evaporation as well. When you dissolve a soluble substance or solute in a solvent, the crystal structure of the substance breaks down into individual atoms, ions, or molecules, which get dissolved in the solution.
When evaporation takes place, the amount of solvent gets reduced. This in turn, causes the excess solute to gather into clusters and crystallize.
Crystallization can be augmented by changing the temperature of the solvent. Solubility can be reduced by lowering the temperature. The rate of crystallization can be increased by changing the nature of the solvent by adding a non-solvent to the solution, which reduces solubility and ensure rapid crystallization.