Hydrate
Hydrate is a term which means different things in
inorganic chemistry and
organic chemistry.
In organic chemistry, a hydrate is a compound formed by the addition of
water to a host molecule. Thus ethanol could be considered to be the hydrate of
ethylene. These substances do not contain water as such, but have their constituents (
hydrogen,
oxygen,
hydroxyl) so arranged that water may be eliminated. Hence, hydrates are derivatives of, or compounds with, hydroxyl.
In inorganic chemistry, hydrates contain water molecules that are either bound to a metal center or crystallized with the metal complex. Such hydrates are also said to contain "water of crystallization". If the water is
heavy water, where the hydrogen consists of the
isotope deuterium, then the term
deuterate may be used in place of
hydrate.
The opposite of a hydrate is an
anhydrate, such substances contain no water or form no water upon heating.
The notation of
hydrous compound, where
n is the number of water molecules per molecule of salt, is commonly used to show that a salt is hydrated. The
n is usually a low
integer, though it is possible for fractional values to exist. In a
monohydrate n is one, in a
hexahydrate n is 6 etc. Such water is also referred to as
water of crystallization.
Examples include
borax,
chloral hydrate,
clathrate hydrates (a class of solid hydrates of gases), and
chalcanthite.
Gas hydrates are clathrate hydrates: water ice with gas molecules trapped within. When the gas is
methane it is called a
methane hydrate.
The presence of
hydrates is quite purposeful in the three fields of endeavour. Generally, in
construction and refractories, inorganic binders are often deprived of water during manufacture. For instance, both in
cement and
gypsum products,
heat is applied to the raw materials. Once water is added on a construction site, the powder is re-hydrated and able to form bonds with other substances that are present. Thus, one goes from powder, to slurry, or paste and then forms
"cement stone".
Water that is
not chemically bound, or converted into hydrates, can come off again as
steam, especially due to the
heat of hydration, with cement products in particular, which undergo an
exothermic chemical reaction with water. Generally, the longer one can keep cementitious products
wet, immediately after placement, the better. The wetter cementitious products are kept, the more water will be converted into hydrates, instead of evaporating off due to the heat of hydration and other environmental influences. Premature drying is a cause for severe concrete problems, such as cracking and shrinkage. The same thing applies to all other cementitious building products, such as spray
fireproofing and
firestop mortars in particular, where the slightest cracking can lead to rejections. The chemically bound water, is the source for
endothermic reactions when exposed to
fire. Fire temperatures in a
building can reach 1100°C, depending on the fuel present and the availability of
oxygen. The presence of hydrates keep the item exposed to the
heat at or below 100 °C, until all the water is spent. Therefore, the more hydrates, the longer the fire-resistance duration. This is what lends fire-resistive characteristics to basic, or "old"
building materials, such as gypsum,
concrete or
plaster. The same thing applies to many high-tech PFP products such as
intumescent and endothermic paints, wraps and tiles, such as those used in
space physics, for re-entry vehicles.
*
Intumescent*
Endothermic*
Ablative*
Passive fire protection
