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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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


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.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
References in periodicals archive ?
However, recent work examining hydration status and the resulting core body temperature in outdoor environments suggests that hypohydration is not associated with increases in core temperature to levels that would be considered excessive.
The division of the participants in the two groups was based on matching hydration status. Participants of the CON group were asked to maintain normal diet and hydration habits and complete the daily fluid chart in the week leading up to post-intervention.
The goal was to quantify hydration status during normal training sessions; therefore, every effort was made to prevent interference in the normal training-day practices and logistical decisions of the participants and coaching staff.
In this regard, the bioelectrical impedance vector analysis (BIVA) emerges as a technique to assess hydration status with no inherent errors of bioimpedance equations or requirements for biological assumptions such as the constant tissue hydration (Lukaski and Piccoli, 2012; Norman et al., 2012).
So the aim of the study was to add an agent to ORT which reduces vomiting and improves hydration status efficiently thus decreasing the need for IV rehydration, hospital admissions and the overall burden of the disease.
Blood and urinary measures of hydration status during progressive acute dehydration.
As combat is frequently in austere conditions and physically demanding, hydration status and strategies are paramount.
* Urine-specific gravity, urine color, and urine osmolality are useful screening measures of hydration status (Adolph, 1947; Armstrong et al., 1998).
Other variables of interest assessed included age, sepsis, hydration status on admission based on the attending physician's criteria, main diagnosis on admission, prehospital and in-hospital treatment with nephrotoxic drugs shown in Table 1 (the operational definitions for nephrotoxic drugs are shown in Table 1 of Supplementary Material available online at https://doi.org/10.1155/2017/5241482), ICU admission after 48 hours of hospital stay, dialysis requirement, length of hospital stay (including days of ICU stay if admitted 48 hours after hospital admission), and in-hospital death.
One way to determine your hydration status is to check the color and volume of your urine.
Stotts, Hopf, Kayser-Jones, Chertow, Cooper and Wu (2009) found that "studies of hydration status in the nursing home population show that residents often do not receive the recommended fluid volume" (2009, p.780).