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processes of the combination of water with chemical substances. There are several kinds of hydration.
Hydration of oxides gives hydroxides, which are alkalis, acids, or amphoteric compounds. Thus the addition of water to calcium oxide gives calcium hydroxide (in technology, this process is known as slaking of lime):
CaO + H2O = Ca(OH)2
In industry, the hydration of sulfur trioxide yields sulfuric acid and that of nitrogen oxides, nitric acid:
SO3 + H2O = H2SO4
3NO2 + H2O = 2HNO3 + NO
Hydration of arsenic trioxide gives weak arsenous acid, which has amphoteric properties:
As2O3 + 3H2O = 2H3AsO3
The hydration of organic compounds takes place at multiple bonds. With cyclic compounds, hydration leads to ring opening. Usually, these reactions take place in the presence of alkalis, acids, or heterogeneous catalysts (catalytic hydration). Hydration of this type plays a large part in preparative organic chemistry and in industrial organic synthesis. Thus the direct hydration of olefins produces alcohols; for example, ethylene gives ethanol:
CH2=CH2 + H2O → CH3CH2CH
Hydration of acetylene (Kucherov’s reaction) yields acetaldehyde (vinyl alcohol, which is unstable, is an intermediate product):
CH≡CH + H2O → [CH2=CH–OH → CH3CHO
Hydration of ketene results in acetic acid and that of ethylene oxide, in ethylene glycol:
In the foregoing examples, the water reacts in such a way that the bond between the hydrogen atom and the OH group is broken.
Many inorganic and some organic compounds produce with water solid crystal hydrates of constant composition, which behave like true chemical compounds. Thus anhydrous copper sulfate, CuSO4, is colorless, but the bright blue hydrate CuSO4 · 5H2O, blue vitriol, crystalizes from its aqueous solutions. Upon heating, blue vitriol first forms light blue CuSO4·3H2O, and then white CuSO4 · H2O; at 258° C the salt is completely dehydrated. Of the same character is the hydration of molecules in solutions with the formation of various hydrates that are in equilibrium with one another and with water, for example, solution of alcohol yields hydrates with three, four, and eight molecules of water. Solution of electrolytes leads to the hydration of ions, impeding association of the latter. To a considerable extent the energy of hydration offsets an electrolyte’s energy of dissociation. Thus hydration is one of the main causes of electrolytic dissociation in aqueous solutions. The formation of crystal hydrates and the hydration of molecules and ions in solutions are special cases of solvation, that is, the addition of the solvent molecule. Hydration also includes processes leading to the binding of water through absorption forces.
In biological systems hydration involves the addition (binding) of water to various substrates in the organism. The water, which enters into the hydration shells formed on hydration, constitutes the principal quantity of the so-called bound water of a cell’s protoplasm. Many biological processes are connected with hydration. Thus the hydration of ions affects their penetration into a cell, while hydration of proteins alters some of their properties, particularly the fermentative activity.
The opposite process of hydration, that is, substances losing bound water, is dehydration. Hydration and dehydration are constantly occurring in metabolic processes in organisms, particularly in water exchange.