Crystallization Definition
Crystallization occurs when materials solidify from liquids, or when they precipitate from liquids or gases. A physical change, such as a change in temperature, or a chemical change, such as acidity, can cause this change.
A crystallization process depends on the size and shape of the molecules involved, as well as their chemical properties. A crystal can be formed from a single species of atom, from different species of ions, or even from larger molecules like proteins. The crystallization process is difficult for some large molecules, since their internal chemistry is not symmetrical or they interact with each other.
A crystal’s smallest unit is called a unit cell. This is the basis on which additional units can be attached to atoms or molecules. Think of it as a building block for children, to which other blocks can be attached. As if you were attaching these blocks in all directions, the crystallization process proceeds. Crystals formed from some materials have different shapes, sizes, and colors, which accounts for the great variety in shape, size, and color.
Crystallization Process
Nucleation
Nucleation is the first step in crystallization. The first atoms in the mass to form a crystal structure become a nucleus, and more atoms organize around it. A small seed crystal is formed as more unit cells assemble around the nucleus. In crystallization, nucleation is extremely important because the nucleus determines the structure of the entire crystal. As the crystal forms, imperfections in the nucleus and seed crystal can lead to drastic rearrangements. Solvents or liquids that are supercooled or supersaturated cause nucleation.
Any liquid on the verge of becoming a solid is a supercooled liquid. A nucleus must form first in order for that to happen. Around this nucleus, crystallization will continue. If atoms or molecules in a liquid are no longer able to bounce off of each other, a nucleus will form. In contrast, they interact with each other and form stable crystal formations. At normal temperatures and pressures, pure elements usually form crystals, but larger molecules may be harder to crystallize.
When a solution is supersaturated, the solvent carrying the crystal is at capacity. As the temperature cools, or the acidity changes, the solubility of atoms and molecules in the solution changes, and the solvent can hold less of them. Thus, they “fall out” of the solution and collide with each other. As a result, nucleation and crystallization occur.
Crystal Growth
In addition to the seed crystal, other molecules and atoms surround the nucleus and branch off from the already established symmetry. Depending on the conditions, this process may happen very quickly or very slowly. It takes minutes for water to crystallize into ice, whereas quartz and diamonds take millennia to crystallize. Crystal structure is determined by the basic formation around the nucleus. From the uniqueness of a snowflake to the clarity of a diamond, this difference in formation accounts for crystal differences.
Crystals can only take a limited number of geometric shapes. Molecules interact and form bonds that determine these properties. Depending on the original nucleus, the bond angles of atoms create the different shapes. There will be a deviation from the typical pattern if the solution or material contains impurities.
The presence of even the tiniest impurity in the nucleus can create a completely unique design, as seen in snowflakes.
Laboratory Uses of Crystallization
The crystallization process is a common and useful laboratory technique. In addition to purifying substances, advanced imaging techniques can be used to understand the nature of the crystallized substances. Solvents can be used to dissolve substances in laboratory crystallization. The material can dissolve with heat and acidity changes. Reversing these conditions causes the materials in the solution to precipitate out at different rates. Crystals of a desired substance can be obtained if the conditions are controlled properly.
A high-energy beam or particle can be shot through a pure substance’s crystal structure by an advanced imaging technique called crystallography. In spite of the fact that there is no visible image created, the rays and particles are diffracted in a specific pattern. A special developing paper or electronic detector can be used to detect these patterns. The pattern can then be analyzed by mathematics and computers, and a crystal model can be developed.
Diffraction patterns are created when electron-clouds within crystal structures redirect particles or beams. During crystallization, these dense areas represent the atoms and bonds of the crystal. In this way, scientists can identify almost any substance based on its crystal structure.
Crystallization Examples
Human time-scale
It takes crystals a long time to form, or they can form quickly. There are many events in nature in which crystallization occurs quickly, so scientists were able to study crystallization. The crystallization of water can be seen in ice and snowflakes, as we have already discussed. The crystallization of honey is another interesting example. Bees regurgitate honey as a liquid into their honeycombs.
During the process of crystallization, sugar molecules within honey begin to form crystals. Look inside an old honey bottle if you have one. It is likely that the liquid will contain small crystals of sugar. Place the honey in the refrigerator to speed up the process. The cooling of the liquid decreases its solubility, causing the sugar to crystallize rapidly.
Geological time scale
Quartz, ruby, and granite form over much longer periods of time, although the process is similar. Under extremely high pressure, these crystals form within the crust and magma of the Earth. Although the process of crystallization is the same, it takes a long time for the conditions and atoms to unite in the right way.
It is possible to replicate these processes in the laboratory, in shorter times, by creating ideal conditions for crystallization. Additionally, laboratories can grow seed crystals, which can be introduced to speed up the production of large batches of crystals at once.
FAQ’s
Crystallization is a physical process in which atoms, ions, or molecules arrange themselves in a highly ordered, repeating pattern to form a solid crystal structure. It involves the formation of stable, well-defined crystals from a solution, melt, or vapor phase.
Crystallization typically occurs when a solution or melt becomes supersaturated, meaning it contains more solute or dissolved substance than it can hold under the given conditions (e.g., temperature, pressure). As the solution or melt cools or evaporates, the excess solute starts to come out of the solution and form solid crystals.
Crystallization has numerous applications across various fields. In chemistry, it is used for the purification of substances, separation of mixtures, and production of pharmaceuticals, chemicals, and materials. Crystallization is also vital in geology for understanding the formation of minerals and gemstones. Additionally, it plays a role in many industrial processes, such as the production of sugar, salt, and semiconductors.
Several factors affect the crystallization process. Temperature is a critical parameter, as cooling promotes crystal formation. The rate of cooling can influence crystal size and quality. The concentration of the solute, solvent properties, presence of impurities, and stirring can also impact crystallization. Additionally, factors like pressure, pH, and seeding (introduction of existing crystals) can influence the outcome.
There are various crystallization techniques employed depending on the specific goals. Some common methods include evaporation, cooling crystallization, fractional crystallization, and freeze crystallization. Other techniques include solvent evaporation, precipitation, and the use of anti-solvents or additives to induce crystallization. Each method has its advantages and is chosen based on the properties of the substances involved and the desired outcome.