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The incorporation of molecular water into a complex molecule with the molecules or units of another species; the complex may be held together by relatively weak forces or may exist as a definite compound.



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):


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.


1. The formation of a compound by combining water with some other substance.
2. In concrete, the chemical reaction between cement and water.
3. The chemical reaction by which a substance (such as portland cement or plaster) combines with water, giving off heat to form a crystalline structure in its setting and hardening.
References in periodicals archive ?
Several methods have been used to measure hydration status during periods of acute fluid loss.
Visiting athletes' hydration status may be compromised through the demands of travel.
Data from three of our previous studies indicates that hydration status is not influential on breath alcohol concentrations or alcohol pharmacokinetics (Irwin, Goodwin, Leveritt, Davey, & Desbrow, 2012; Irwin, Leveritt, Shum, & Desbrow, 2013, 2014).
Hydration status can by measured using refractometry, with many studies validating the use of refractometry via the measurement of urine specific gravity (Armstrong et al.
At the end of the exercise, soldiers were found to have good immune and hydration status, with unchanged physical and mental performance, albeit with about a 5% reduction in weight and reduced status of some storage nutrients.
Pain and its treatment can have dramatic effects on a resident's nutrition and hydration status.
Inform athletes on how to monitor their hydration status.
It interactively manages hydration status through real-time feedback on fluid consumption relative to an individual's pre-set personal hydration goals.
People can check their hydration status by monitoring the colour of their urine.
In addition, urinary specific gravity was measured in order to quantify hydration status.
Take special care with drugs that are sensitive to hydration status, such as lithium, she said.
5%, decrements in physical and mental performance were not observed and good immune and hydration status were maintained.