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What is crystallization?

Crystallization is the process of formation of solid crystals precipitating from either a solution or directly from a gas phase.

In other words, crystallization is defined as a process by which a chemical is converted from a liquid solution into a solid crystalline state. In industrial applications, the widespread use of crystallization due to the fact that crystallization acts as both a separation and purification step.

Needless to say, almost all chemical processes utilize at least one crystallization step (if not both). But crystalization itself is not a chemical reaction. The process of crystallization is actually a physical change.

In chemistry for example, recrystallization is a technique used to purify chemicals. By dissolving both impurities and a compound in an appropriate solvent, either the desired compound or impurities can be removed from the solution, leaving the other behind.

Crystallization occurs in two major steps, namely:

  1. Nucleation, and
  2. Crystal Growth

In nucleation, there is appearance of a crystalline phase from either a super-cooled liquid or a supersaturated solvent. Molecules gather together in clusters in a defined manner. These clusters need to be stable under experimental conditions to reach the “critical cluster size” or they tend to redissolve. It is at this point in the crystallization process that defines the crystal structure.

The second step, known as crystal growth, is the increase in the size of particles and leads to a crystal state. Here, the nuclei that have successfully achieved the “critical cluster size” begin to increase in size. Crystal growth is a dynamic process, with atoms precipitating from solution and becoming redissolved. Supersaturation and supercooling are two of the most common driving forces behind crystal formation.