In physics and chemistry, the word crystal is used in reference to any solid in which all atoms are arranged in an orderly manner. The surface structure of crystals are symmetrical. The atoms within a crystal are arranged into unit cells. Each of these individual structural units repeat in all directions, creating symmetry. Crystals are found in nature but can also be recreated in a science lab using different techniques.
Chemists use this technique of crystallization to purify solid compounds. Compounds that are soluble tend to dissolve in hot liquids to a greater degree than in cold liquids. The compound melts and is dissolved in a hot solution. On cooling, the compound will crystallize out in pure form. In nature, this form of crystallization is found on the surface of the ocean floor when magma cools and after volcanic eruptions when lava cools.
In solution crystallization, water is often used to separate crystals from a solvent. A solution is first saturated to its maximum potential by adding the compound to 1 cup water. The compound is added while stirring and not added further when it begins to sink to the bottom and no longer dissolves. The contents of the cup are then poured into a funnel, which is lined with filter paper. The filter paper is then removed and allowed to sit until the water evaporates and crystals begin to form.
Crystals can be grown from vapors through sitting and hanging drop crystallization. In the sitting drop technique, a small droplet of 1 up to 40 micro liters is mixed with crystallization reagent. The concentration of the droplet is initially less than that in the reservoir. The reservoir pulls water from the droplet in vapor phase over time. An equilibrium exists between the drop and reservoir, slowly saturating the sample and causing crystallization.
Factors that Affect Crystallization
The factors that affect the rate of crystallization and size of crystals formed include the presence of impurities and additives. The more time allowed for evaporation, the larger the crystals will grow. If less time is given by speeding up the process through heating, smaller crystals will be formed.
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