Crystallization is the chemical process of transforming a gas , liquid , or solution into a network of bonds that results in a crystal lattice. For example: frost formation, silicon purification, aspirin manufacturing.
This process can be used to separate solid components from a solid-liquid mixture, which is why it is often used in the purification of solids . The method involves transferring the component from the liquid phase to the solid phase in the form of precipitated crystals.
Thus, crystallization can be used to separate components of a homogeneous mixture , for example, salt water (NaCl). The separation can be accomplished through several types of crystallization, including selectively altering the temperature or pressure of the mixture, as well as the addition of other substances .
The shape, size, and quality of the crystals obtained will depend on the conditions and the length of time they are allowed to form. If the crystals are too small, they may become covered with impurities, and if they are too large, the impurities may become trapped within the crystal lattice.
Crystals are solid formations that exhibit a well-defined diffraction pattern. They are common in nature and are classified according to their composition as:
- Solid crystals. These are solid crystalline formations. They have an ordered structure, unlike glass and amorphous compounds (without a regular or ordered shape), which have disordered structures.
- Luminous crystals. These are formations with structures whose degree of arrangement is intermediate between crystalline and amorphous. They are used to make screens.
- Ionic crystals. They are composed of cations and anions , held together by electrostatic force.
- Covalent crystals. They are made up of atoms held together by covalent bonds . They are very hard.
- Molecular crystals. They are formed by molecules held together by Van der Waals forces or hydrogen bonds, not by chemical bonds . They have relatively low melting points.
- Metallic crystals. They are made up of atoms of the same metal and almost all are good conductors of electricity.
It may be useful to you:
- Decantation
- Magnetization
- Sieving
Examples of crystallization
- Frost formation. On particularly cold days, water vapor in the air can crystallize on cold surfaces such as glass or certain metals, similar to the formation of snow. This is known as frost, but it actually consists of water crystals of a very regular and well-formed structure.
- The freezing of water. Although ice itself is not a crystal, during the initial stages of water freezing, it is possible to see the formation of dendrites and other submerged structures in the container that look very similar to crystals.
- Evaporation of salt water. This procedure is very common both in obtaining salt crystals and in desalination of water. When the liquid boils, it turns into a gaseous state , and the salts dissolved in it remain, their molecules reattaching in the form of salt crystals at the bottom.
- The manufacture of aspirin. Acetylsalicylic acid, the active ingredient in this popular remedy, is obtained through an esterification reaction that crystallizes in the presence of ethanoic anhydride and sulfuric acid.
- Winterization of oils. This process is useful for obtaining clearer, lower- density oils . It involves rapid and sustained cooling of the oil to cause the crystallization of stearins, saturated glycerides, waxes, and other unwanted substances. Once these have formed solid crystals, the oil is filtered and centrifuged before they can regain their liquidity and are extracted from the mixture .
- Sugar crystallization. Sucrose and other sweeteners (commercially presented as crystals to be dissolved in beverages) have undergone a crystallization process from the sweet syrup from which they are obtained. The mixture is then centrifuged to separate the crystals from the honey. Brown sugar (not white sugar) is actually sugar in its first stage of crystallization (unrefined).
- Covalent carbon crystals. Subjected to enormous underground pressures and slow metamorphosis processes , carbon can transform into any of its three allotropes: coal, graphite, or diamond. The latter is precisely an example of crystal, whose atoms are so tightly bound that they exhibit a recognized hardness and extremely low melting point .
- Regressive sublimation. Certain solids that, when exposed to heat, transform into a gaseous state ( sublimation ) can later return to their physical form as crystals when exposed to a decrease in temperature, in what is called reverse sublimation. In the process, the impurities in the solid will be lost, and pure crystals will be left in their place. This process is useful for purifying iodine or sulfur, for example.
- Silicon purification. Although silicon does not sublimate, it can be purified by melting and then selectively cooling it to remove soluble impurities from the high-purity silicon single crystals, which are then used in the semiconductor industry .
- Crystallization of benzoic acid. This crystallization process occurs from a solution of benzoic acid in acetone, with the simple addition of water. The interaction between the two solvents creates a new mixture, and the benzoic acid crystallizes at the bottom of the container.
- Marine calcareous formations. Such as those of mollusks , corals, and bivalves, which, through the action of certain proteins , can not only precipitate but also mold the creation of calcite or quartz crystals on the rock on which their colony will form.
- The formation of molecular crystals. In substances such as sulfur dioxide (SO 2 ), Van der Waals forces and hydrogen bonding lead to the formation of molecular crystals, which are usually brittle and meltable below 100 °C.
- Silver crystals for film. Obtaining silver crystals is useful for certain implements in the early (non-digital) film or photographic industry, as they are light-sensitive and allow the substance to rearrange itself according to the light impression through the lens. They are obtained from chemical compounds such as silver bromide, chloride, or iodide.
- Calcium oxalate crystals. These crystals form from the deposition of salts and calcium in the kidneys, where they oxidize and form small, dark stones that must then be painfully expelled with urine. This is a common kidney condition known as kidney stones.
- Uric acid crystallization. This is the phenomenon associated with the disease known as gout , in which uric acid crystals form in the joints, causing pain and decreased movement . It can be a consequence of excessive purine intake or kidney failure of varying severity.
