Everything about Hydrates totally explained
Hydrate is a term used in
inorganic chemistry and
organic chemistry to indicate that a substance contains
water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood.
Chemical nature of hydrates
In organic chemistry, a hydrate is a compound formed by the addition of
water or its elements to a host molecule. For example,
ethanol, CH
3—CH
2—OH, can be considered as a hydrate of ethylene, CH
2=CH
2, formed by the addition of H to one C and OH to the other C. A molecule of water may be eliminated, for example by the action of
sulfuric acid. Another example is
chloral hydrate, CCl
3—CH(OH)
2, which can be formed by reaction of water with
chloral, CCl
3—CH=O.
Other molecules have been labeled as hydrates for historical reasons.
Glucose, C
6H
12O
6, was originally thought of as C
6(H
2O)
6 and described as a
carbohydrate, but this is a very poor description of its
structure as known today. And
methanol is often sold as “methyl hydrate”, implying an incorrect formula CH
3OH
2, although the correct formula is CH
3—OH.
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" or "water of hydration". 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.
A colorful example is
cobalt(II) chloride, which turns from blue to magenta (red) upon
hydration, and can therefore be used as a water indicator.
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's 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 decahydrate,
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's called a
methane hydrate.
A hydrate which has lost water is referred to as an
anhydride, and can normally lose further water only upon strong heating, if at all. A substance which contains no water is referred to as
anhydrous.
Applications of hydrates
Construction
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's
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 shrinking.
Passive fire protection (PFP)
Avoiding premature drying is important to all other cementitious building products, particularly spray
fireproofing and
firestop mortars, where the slightest cracking can lead to rejection. The chemically bound water is used up by
endothermic reactions when exposed to the
heat of a
fire. Fire temperatures in a
building can reach 1100°C, depending on the fuel present and the availability of
oxygen, but the hydrates keep the temperature of the item 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, like
gypsum,
concrete or
plaster.
Fire-resistance duration is important 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.
Further Information
Get more info on 'Hydrates'.
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