Crystallization

Crystallization is the process by which a substance transforms from a gas, liquid, or solid into a crystal. Everyday examples of crystals include table salt, sugar, and snow. Diamonds and many other gemstones are also crystals.

When crystallization occurs, the molecules, atoms, or ions of a substance form into fixed, symmetrical patterns called lattices. A crystal lattice is a three-dimensional configuration of points connected by lines used to describe the precise arrangement of atoms in a crystal. Each point characterizes one or more atoms in the crystal.

Crystals are divided into seven systems (cubic, tetragonal, orthorhombic, monoclinic, triclinic, trigonal, and hexagonal), which are determined by the symmetry of a crystal's lattice. Crystalline describes a state in which the molecular arrangement becomes highly regular and allows the formation of distinct external crystals. Fully formed solid crystals usually have smooth surfaces. Since all crystals grow under different conditions, no two crystals look identical.

Crystallization Process

The study of crystals, known as crystallography, did not experience any major progress until the seventeenth century. In 1669, Danish geologist Nicolaus Steno made an important discovery about quartz crystals. He found that the angle between any two surfaces of quartz crystals was the same, regardless of the size of the surface. Steno's work led to the formation of the law of constancy of interfacial angles, which states that the angles between all corresponding crystal surfaces are constant, regardless of crystal size or surface development. In the nineteenth century, investigators concluded that all crystals consist of smaller units of crystals shaped exactly like the whole crystal. This conclusion helped form a better understanding of crystal lattices, which are also called space lattices. Space lattices are the regular array of points about which the atoms, ions, or molecules within a crystal are centered. All the points in the lattice are identically structured so the environment looks the same from any point.

Crystallization develops in two stages: nucleation, which is the formation of a seed crystal, and crystal growth. Before nucleation can take place, a liquid must be supersaturated, which means that the mixture should contain more dissolvable material than can be dissolved by a solvent, which is a substance that dissolves things. This is achieved by changing the temperature of the substance. Nucleation is either homogeneous or heterogeneous. Homogeneous nucleation is spontaneous, whereas heterogeneous crystallization is caused by foreign particles such as dust or dirt. Once this process is completed, crystals begin to grow. Crystal formation depends on several factors, including temperature, pressure, and surface tension, or the strength of a solvent's surface. Each crystal grows as its lattice planes pile together, eventually forming one large unit. The growth habit of a crystal determines its shape. For instance, if crystal growth is vertically stunted, the crystal usually grows flat and plate-like. Vertical, elongated growth produces a needle-shaped crystal.

Several natural and artificial crystallization methods exist. Crystallization can occur if a liquid is cooled below its melting point, which is how ice forms. Crystallization can also be achieved through evaporation. Chemists use crystallization to purify solid compounds. Compounds are easily dissolved in hot liquids. Cooling a hot liquid makes the compounds less dissolvable, allowing crystals of pure compound to form. The crystal growths, free of chemical impurities, can then be separated from the solution. This method is used when extracting salt crystals from sea water, for example. Crystals are also grown for experimental or industrial purposes. Scientists can control the growth of the crystal and tamper with its molecular structure by introducing foreign substances into the seed crystal. Electrocrystallization is an example of an experimental crystal growing process that produces protective coatings for industrial metals.

Crystallization in Nature

Crystallization is observable in nature. For example, when liquid rock, or magma, begins to cool, crystals can form. This process is called magma crystallization. Precious gems such as diamonds, rubies, and emeralds are also the result of crystallization. Crystallization is the reason snowflakes and icicles form. Honey can also become crystallized if left in a cool environment.

The four basic types of crystalline solids are ionic, covalent, molecular, and metallic. Ionic crystals include ionically charged substances such as sodium chloride, or salt. A diamond is an example of a covalent crystal, which is connected by covalent bonds. Organic compounds such as solid phosphorus can transform into a molecular crystal, which are joined by weak molecular forces. Finally, metallic crystals include metals such as copper and are composed of positively charged ions.

Polymorphism is the phenomenon of a chemical species having multiple crystalline forms. For example, the chemical structures of carbon graphite, pencil carbon, and carbon created diamonds are identical, but arrange themselves in different forms.

Bibliography

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