We explain what crystallization is and what this chemical process consists of. Also, the methods it uses and examples of crystallization.
What is crystallization?
It is known as crystallization to a chemical process in which a gas, a liquid or one dissolution, in a set of solid crystals. These crystals are formed by an ordered set of rigid molecular bonds that are pure in their elemental nature. Because of this, crystallization can be employed as a method to separate solid components from some homogeneous mixture, that is, it can be used as a purification method.
There are different methods of crystallization, which can employ the selective alteration of the physical conditions of temperature or from Pressure, as well as the addition of certain chemical substances. The shape, size and quality of the crystals thus obtained will depend on the specific conditions in which the process occurs and the time during which it is allowed to occur.
The crystals obtained by this method They are solid formations, endowed with a very well defined diffraction pattern (a phenomenon that consists in the deviation of a wave when it passes through an obstacle through a small hole and then propagates in all directions behind this hole). Depending on the conditions in which crystallization takes place and the type of substance to be crystallized, the crystals obtained will have a color specific, a specific transparency and a defined geometry.
Crystals are common in mineral nature and are classified according to their properties in:
- Solid crystals. They are the most numerous. With the exception of glass and some solid amorphous substances (solid substances in which their particles they are not ordered, that is, they lack shape), almost all solid matter is in a crystalline state. For example: salt, sugar and some precious stones.
- Luminous crystals. They are considered as liquids and are often used in electronic equipment displays.
- Ionic crystals. They are hard and brittle. The cations and anions that form them are of different sizes and, in general, these crystals have high melting points. For example: potassium chloride (KCl) and zinc sulfide (ZnS).
- Covalent crystals. The atoms that compose them are united by covalent bonds forming a three-dimensional network. For example: graphite and diamond.
- Molecular crystals. Its particles are bound by Van der Waals forces (attractive or repulsive forces between molecules, interactions that are less than those corresponding to a Chemical bond) and / or hydrogen bridge interactions (attractive interactions between a hydrogen atom bonded to an electronegative atom, with another electronegative atom that belongs to another molecule, these interactions are also less than those corresponding to a chemical bond). These compounds are usually brittle and their melting points are below 100ºC. For example: tetraphosphorus (P4) and diiodo (I2).
- Metallic crystals. Metallic crystals are composed of atoms of the same metal. They are generally very dense and their properties (such as melting point and hardness) vary according to the type of metal. On the other hand, these compounds They are good drivers of the heat and the electricity. For example: nickel (Ni), iron (Fe) and cobalt (Co).
Examples of crystallization
Water vapor in the air can crystallize directly on cold surfaces.
- Frost formation Under certain conditions of humidity environmental, water steam of air can crystallize directly on cold surfaces (such as glass or metals) and form snow-like structures called Frost. Some freezers tend to form frost as well. These are crystals of Water, whose constitution is very regular and very well formed.
- Freezing the water. Ice is frozen water and as such it is not a crystal. But during the first phases of freezing of this liquid, you can see how dendrites (crystals in which the crystal orientations are all different) and other submerged crystal structures emerge.
- Sea water evaporation. To obtain salt crystals, as well as desalinated water, water taken from the sea. In this way, the liquid is transformed into a gas (water vapor), leaving the salts that were dissolved in the container, which remain as perfect saline crystals.
- Silver crystals for photography. Silver crystals are useful for certain artifacts of the film industry or old photography (not digital, obviously) since being sensitive to the light, these crystals are rearranged in front of the light, thus copying the light impression. To obtain them, compounds such as silver bromide, chloride or iodide are used.
- Calcium oxalate crystals. Formed by the accumulation of salts and calcium in the kidneys, these crystals are usually painful for the human being, and sometimes a surgical intervention is required to extract them, since they hinder the normal expulsion of urine. They are in the form of small dark stones, known as kidney stones, or also "stone" or "grit" in the kidneys.