Crystallization is a solid state (natural or man-made) process in which atoms or molecules are highly organized into a structure called crystal. Some ways of crystal formation are by precipitation from solution. After the hot saturated solution is cooled, the solute is precipitated in the form of crystals.

The process of crystal formation in solution is called crystallization. There are generally two crystallization methods: one is the evaporation solvent method, which is suitable for substances whose temperature has little effect on solubility. This method is used to "sun salt" in coastal areas. The other is the cooling heat saturated solution method. This method is suitable for substances with increased temperature and solubility. For example, in the salt lakes in the north, the temperature is high in summer and no crystals appear on the lake; every winter, when the temperature drops, soda (Na2CO3·10H2O), Glauber's salt (Na2SO4·10H2O) and other substances are precipitated from the salt lake. In order to obtain larger intact crystals in the laboratory, a method of slowly lowering the temperature and slowing down the crystallization rate is often used.
People cannot see the macroscopic phenomenon of the dissolution and crystallization of the substance in the solution at the same time, but there are actually two reversible movements of the dissolution and crystallization of the particles that make up the substance in the solution at the same time. By changing the temperature or reducing the solvent, the crystallization rate of the solute particles at a certain temperature can be greater than the dissolution rate, so that the solute will crystallize out of the solution.
The formation of crystals can be divided into two types. The first type of crystals consists of cations and anions, and is also called salts, such as sodium acetate. The second crystal is composed of uncharged species, such as menthol.
The formation of crystals can be achieved by various methods, such as: cooling, evaporation, adding a second solvent to reduce the solubility of the solute (known as anti-solvent or trapped technology), solvent stratification, sublimation, changing cations or anions , And other methods.

Common methods of crystallization
Evaporative crystallization
Evaporative crystallization refers to the process of removing the solvent from the solute by heating the solvent in the solution, and is the process of polymerizing the solute to become a solid (crystal). Heating to evaporate the solvent will change the solution from unsaturated to saturated, and continue to evaporate, and the excess solute will be crystallized out, which is called evaporative crystallization.

Cooling crystallization
Cooling crystallization is a method of crystallizing saturated solutions. In essence, the solute is precipitated out of the solution in the form of crystals by reducing the temperature (applicable to substances whose solubility increases significantly with the increase of temperature). Generally refers to the process of reducing the temperature of a saturated solution at a relatively high temperature to make it precipitate crystals.

Recrystallization
Recrystallization is a process in which crystals are dissolved or melted in a solvent and then recrystallized from a solution or melt. Also called recrystallization. Recrystallization can purify impure substances or separate the mixed salts from each other. The effect of recrystallization is closely related to the choice of solvent. It is best to choose a solvent that is soluble to the main compound and slightly soluble or insoluble to impurities. After the impurities are filtered out, the solution is concentrated and cooled to obtain a pure substance. Two salts mixed together, if their solubility in a solvent changes greatly with temperature, such as a mixture of potassium nitrate and sodium chloride, the solubility of potassium nitrate increases sharply with the rise of temperature, and the temperature rises High has little effect on the solubility of sodium chloride. Then, the mixture solution can be evaporated and concentrated at a higher temperature. The sodium chloride crystals are precipitated first, and the mother liquor after removing the sodium chloride can be concentrated and cooled to obtain pure potassium nitrate. Recrystallization often needs to be performed many times to obtain a better purification effect.
The highly soluble solution obtained by recrystallization may be an impurity or a required solute. It has a wide range of applications.
A method of increasing the crystallinity of the polymer by selective chemical reaction of the amorphous region in the crystalline polymer. For example, a degradation method is used to cause chain scission in the amorphous region to promote the rearrangement of random or orderly molecular chains, increase orderliness, and thereby improve crystallinity.

Crystallization Chemistry
A chemistry that studies the relationship and laws between the structure of a crystal and its chemical composition and properties. It mainly includes the purposeful accumulation of materials with crystal structure, and the use of these materials in conjunction with other data to clarify and solve related chemical problems. In-depth disclosure of the intrinsic relationship of crystals, so as to fully grasp the laws, to achieve the preparation of crystal materials with specified properties such as semiconductors. It is of great significance in scientific research and industrial production.

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