Dehydration
Dehydration is the removal of water from a compound, set of compounds, or other material. Dehydration refers not only to the removal of pre-existing water molecules from a solution, but also to the chemical formation of water by its removal from a compound to form a new compound.
The simplest form of dehydration is evaporation, in which water spontaneously leaves the liquid phase. The evaporation rate can be increased with heat. As water leaves a solution, the remaining solution becomes increasingly concentrated. At sufficiently high concentrations, solute molecules or ions may exceed their solubility, and begin to crystallize. This process is one of the most common means of obtaining crystals, both in the laboratory and in nature.
Dehydration for the purpose of crystallization may be aided by vacuum, which lowers the vapor pressure above the solution to increase the rate of evaporation. Freeze-drying uses evacuation and cold to prevent deterioration of solutes during dehydration. Freeze-drying is an especially effective means of food preservation, since the water content can be brought down so low that most microorganisms cannot grow on the food.
For substances with lower boiling points than water, heating can be used to drive off the more volatile substance, which can then be collected by condensing it. This process is known as distillation, and is used to purify many low-boiling organic compounds, such as ethanol. Ethanol produced by fermentation is at most a 20% solution in water. Distillation can raise its percentage up to 95%.
Simple distillation cannot completely dehydrate ethanol, because it forms an azeotrope with water. An azeotrope is a mixture with a constant boiling point that cannot be separated by distillation. By adding a small amount of other organic liquids, the ethanol can be distilled to 100%.
Gases and non-aqueous liquids can be dehydrated by passing them over or through a hygroscopic substance, one that readily absorbs water. Solid calcium chloride is used for this purpose both in the laboratory and in the home, where cans of anhydrous ("without water") calcium chloride may be placed in damp closets, for instance, to prevent mildew. Anhydrous sodium hydroxide is another common laboratory desiccant, or substance that removes water. "Molecular sieves" made from zeolite clays are even more effective in many applications. The high internal surface area of these clays allow them to bind tightly with very large amounts of water.
Dehydration, in the sense of formation of a water molecule through chemical reaction, is an important part of organic chemistry and biochemistry. Alcohols such as ethanol (H3CCH2OH) can be dehydrated in this sense by reaction with heat in the presence of sulfuric acid, which acts not only as a catalyst, but as a hygroscopic medium to remove water as it is formed. The product, ethene (H2C=CH2) forms by removal of the OH from one carbon and an H from the other, leaving a double bond uniting the two carbons. This is one of the most common ways of synthesizing alkenes, double-bonded hydrocarbons.
Dehydration is perhaps the most common and vital reaction in the synthesis of biological macromolecules, such as proteins, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), carbohydrates, and fats. These four types of molecules are polymers, each formed from smaller building blocks, or monomers, which become linked by dehydration.
The basic reaction is A OH + HO B A O B +H2O. (For proteins, the basic reaction is A OH + H B A B +H2O.) For proteins, A and B represent amino acids; for DNA and RNA they represent nucleotides; for carbohydrates they represent single sugars; and for fats, they represent a fatty acid and a glycerol.
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