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Offline madscientist

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hydrates??
« on: 14/03/2006 12:15:06 »
hi there..

anybody can tell me what happens to hydrates when they are dissolved in water??for ex. C2O4.2H2O,dissolved in water.I know this is such a silly question but have been lingering in my head too much for so long.


 

Offline Ophiolite

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Re: hydrates??
« Reply #1 on: 20/03/2006 12:30:24 »
The hydrate decomposes to water and the entrained gas. The latter then dissolves in the water, or is released, depending upon temperature, pressure and saturation.

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Offline DrDick

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Re: hydrates??
« Reply #2 on: 31/03/2006 21:12:29 »
Actually, hydrates are solid compounds with associated water molecules.  So they do dissociate, with the solid compound dissolving in water and the associated water molecules joining the bulk water.

By the way, I think you meant H2C2O42H2O (oxalic acid dihydrate).  In this case, the oxalic acid (H2C2O4) dissociates from the waters of hydration, then (as an acid) partially dissociates itself, into H+, HC2O4- and C2O4-2 ions.

A related species is the clathrate, in which a gaseous molecule exists in a cage of water (I think this is what Ophiolite was thinking of).  Methane clathrates are found in high pressure locations (e.g., the bottom of the ocean).  The rapid decomposition of methane clathrates was even the premise behind a science fiction book I read about 12 years ago called, "The Mother of Storms", by John Barnes.  Methane is a potent greenhouse gas, so caused rapid environmental changes to occur around the world.

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Offline Ophiolite

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Re: hydrates??
« Reply #3 on: 01/04/2006 00:14:08 »
quote:
Originally posted by DrDick
A related species is the clathrate, in which a gaseous molecule exists in a cage of water (I think this is what Ophiolite was thinking of).  Methane clathrates are found in high pressure locations (e.g., the bottom of the ocean).
You are making a distinction (between hydrates and clathrates) that I am unfamiliar with. It appears the USGS is unfamiliar with it also.
http://marine.usgs.gov/fact-sheets/gas-hydrates/title.html

Perhaps this reflects a difference between geologists and chemists. What do you think?

for what it's worth I think hydrates are a clathrate that is based on a water 'cage'. Other clathrates have different 'cages'.

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« Last Edit: 01/04/2006 00:16:17 by Ophiolite »
 

Offline DrDick

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Re: hydrates??
« Reply #4 on: 03/04/2006 17:17:38 »
I had to go back and check my chemical dictionary (1).:)

Both "hydrate" and "clathrate" are (overlapping) subsets of the term "solvate".

A solvate is a general case where a small species (ionic or molecular) is enclosed in a cage of solvent molecules. (not in the dictionary - this is my definition)

A hydrate is "a substance formed by combination of a compound with water."  (1)

A clathrate is "a solid mixture in which small molecules of one compound or element is trapped in holes in the crystal lattice of another substance." (1)

So, while something may be both a hydrate and a clathrate (as in the methane clathrates/methane hydrates), they don't have to be.  Other solvents (such as quinol) can form the solvent cage around the solute.

Personally, I usually think of hydrates as things like Na2SO410H2O, where Na2SO4 aborbs water from the environment and can lose waters of hydration through heating.

To be honest, I generally assume that clathrates are also hydrates, just a rare form of hydrate.  So when I hear the term hydrate, I usually think of the Na2SO4 example.

(1)  "A Dictionary of Chemistry", Oxfort University Press, 1996
 

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Re: hydrates??
« Reply #4 on: 03/04/2006 17:17:38 »

 

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