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What is crystallization?

The crystallization is a process of forming a solid highly ordered structure. This solid, known as a crystal, can be obtained from a liquid, a solution or a gas, depending on the crystallization method that is carried out.

In general, crystallization is a slow process, since substances (atoms, ions, molecules) require time to be properly ordered. Gems and minerals, for example, crystallize very slowly from lava or salt water, fluids where the entrained metal ions gather over time.

However, not all crystallizations are as slow as with eternal diamonds. Some take place in periods of hours or seconds inclusive. That is, we can find small or microscopic crystals, each with its own chemical nature, that appear in front of our eyes.

Crystallization is the basis for the existence of many natural (minerals, sugars, salts, etc.) and artificial (synthetic gems, alloys, etc.) solids. Therefore, it is common to find crystals everywhere, especially when it comes to rocks, or the snow and ice of the polar caps.

Solid crystals

Solid crystals are all those that correspond to the ordered forms of solid substances. That is, if the chemical structure of a solid is ordered, then we speak of a solid crystal. Now, in the following subsections we will see that not all solid crystals are chemically the same.

Liquid crystals

Liquid crystals are substances that exhibit both liquid (fluidity) and solid (rigidity) properties without melting or freezing. They are generally organic in nature, and have technological applications such as in touch screen design.

Ionic crystals

Ionic crystals are solid crystals made up of ions: atoms or molecules with positive charges (cations) or negative charges (anions). For example, crystals of potassium nitrate (KNO 3 ), a salt, are ionic as they are made up of K + cations and NO  anions .

These crystals are hard and brittle. Why? Because when pressed, ions of equal charges approach and repel each other with great force, which fractures the crystal. They also have high melting and boiling points, because the heat supplied has to be high enough to separate the ions from each other.

Covalent crystals

Covalent crystals are those where the components are held together by covalent bonds. One consequence of this is that ordered, three-dimensional networks are formed. Examples of these crystals are quartz, diamond and graphite.

Molecular crystals are the solid ordered forms that molecules can reach. Therefore, instead of ionic forces or covalent bonds, the components are held together by interactions or Van der Waals forces (also called intermolecular forces).

The examples of molecular crystals are almost limitless. Some of them are: ice (water), iodine (I 2 ), sugar, tetraphosphorus (P 4 ), etc.

Metallic crystals

Metallic crystals are those composed of neutral metallic atoms. For example, gold (Au), silver (Ag), and sodium (Na) have their characteristic metallic crystals, varying the size of the atoms and the nature (or strength) of their interactions.

Crystallization methods

Having mentioned the types of crystals that exist, it will be seen that these can be obtained by different crystallization methods. Some of them, the simplest and most intuitive, are the following:

Cooling

When a liquid freezes it becomes a solid state . If this solid is ordered, then we will have a crystal. In the same way, some gases when cooling rapidly are able to crystallize. Said cooling must be controlled, so that the components that make up the glass can be ordered in time.

Evaporation

Another way to achieve crystallization, as far as solutions are concerned, is by evaporation of the solvent. For example, if we evaporate sea water, dissolved salts will crystallize as sediment (at the bottom or on the walls of a container). Once the liquid has been removed, the components of the crystal will be grouped in an orderly fashion to form the crystal.

Chemical reaction

Chemical reactions can also cause crystallization. If two components of a crystal, which are initially separated, are suddenly mixed, they will bind strongly to each other and form a solid, which will later receive a treatment (drying) and will end up defining a crystal.

Examples of crystallization

Stalactites

The stalactites that “hang” from the cavernous ceilings are also examples of natural crystallization

The slow but continuous crystallization of calcium carbonate (CaCO 3 ) during the dripping in the ceilings of the caves, gives rise to the formation of stalactites. In the process, the Ca 2+ and CO 2- ions meet and group together while the water evaporates.

Stalagmites

Stalagmites are structures that can be found in caves, specifically in soils (growing pointed upwards). They are composed of calcium compounds and are formed from the crystallization of calcium salts found in the water that falls from the ceilings of caves.

Wine glasses

It is common to find a reddish powder under the cork or at the bottom of the glasses, especially with red wines. Said solid is actually a tartrate salt that crystallizes over time, and in the presence of calcium and sodium ions.

Snow formation

Liquid water when frozen forms ice crystals. When these ice crystals are very small, and derive from the droplets suspended in the clouds, then the formation of snow takes place. In snowfall, microscopic ice crystals precipitate, which are deposited on the surroundings as if it were white sand.

Sugar crystals

Another example of a molecular crystal is that of sugar. Instead of water molecules, we have sucrose molecules. Sucrose molecules are held together by Van der Waals forces, which define the sweet, colorful crystals we are used to seeing in pantries.

The crystallization of sugar is one of the most common chemical experiments: a sugary solution is heated and, once cooled, a thread dipped in sugar is deposited. After a while, large and showy sugar masses will have crystallized on the thread.

Copper sulfate crystals

The crystallization of copper sulfate is one of the most attractive that can be seen in laboratories

Similar to the experiment with sugar crystals, the crystallization of copper sulfate crystals is also interesting because of their bluish hues. Again, while the water evaporates, the ions Cu 2+ and SO 2- meet, and in the process they retain some water molecules to form the hydrated salt CuSO 4 · 5H 2 O.

Salt

Salt is the most common example of crystallization. This can be formed both naturally (such as sea salt) and artificially (as is the case with table salt).

Diamond

Diamond is a gemstone that is formed from the crystallization of pure carbon. This is the hardest known material on the planet. Its formation can be natural, as is the case with diamonds found in mining deposits, or synthetic.

Ruby

Ruby is a reddish crystal that is formed from the crystallization of aluminum oxide (coridon).

Quartz

Quartz is a gem that is formed from the crystallization of silicic anhydride. It is one of the most abundant minerals in rocks and its color is variable.

Peridot

Also called olivine, this gemstone is formed thanks to the crystallization of iron and magnesium. It is greenish in color and is usually diamond-shaped.

Silicates

Silicates are materials created by the crystallization of silica and other elements (iron, aluminum, calcium, magnesium). They are present in all rocks.

Candies

Candies are made with sugar crystals, so it can be said that two crystallization processes are involved: the first for the formation of sugar and the second for the formation of molasses.

Creamy ice-cream

Creamy ice cream contains a series of crystals that give it the final smooth texture. Among the crystals that creamy ice cream contains, lipid crystals (formed from fat) and ice crystals stand out. It should be noted that some ice creams also contain lactose crystals.

In this sense, ice cream is obtained through various artificial crystallization processes (one for lipids, one for ice and one for lactose).

Other examples

  • Preparation of sugar crystals around a string or string and a supersaturated sweet solution
  • Formation of sugar crystals from honeys deposited at the bottom of their jars
  • The growth of kidney stones, which consist of the essence of calcium oxalate crystals
  • The crystallization of minerals, including gems and diamonds, over the years, whose shapes and edges are a reflection of their orderly internal structures
  • Deposition of hot metal vapors on cold bars as supports for the growth of their crystals.

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