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Copper sulphate, in its hydrated form, is crystalline, whereas the anhydrous form is amorphous.

Gypsum has a similar story-- on heating the crystalline dihydrate we get an amorphous hemihydrate. (Gypsum in fact has two different crystalline forms of the dihydrate, and has an anhydrous form as well).

I've never really grasped how the water makes it crystalline. I suspect it has to do with the relative sizes of the compounds being re-compensated by ligands, but I'm not sure. How does the water affect/create the crystalline properties?

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[Manishearth, I'm not sure I'm even answering the right question below. After reading your question more carefully, I think you were more interested in how water molecules help crystals form, rather than whether or not the anhydrous forms can form crystals also (versus being strictly amorphous). So, oops. Maybe some one else has a good answer on the water molecules part?]

I suspect the answer is an issue of process rather than of fundamentals.

That is, if you could find a non-aqueous medium that readily dissolves the anhydrous form of some salt, my very strong suspicion is that you could grow excellent clear crystals of it that would look almost nothing like the hydrous crystalline forms.

The problem for metallic-anion salts in particular is that finding a liquid that dissolves the salt without simultaneously forming strong coordination complexes with it is going to be tricky, since I'm guessing it's usually the coordination effect that help ionize the units of the material to make it soluble!

Here's a wild guess: Someone, somewhere, has likely done research on how to grow crystals of substances such as anhydrous $\ce{CaSO4}$ (hmm, why does this phrase "dead burnt plaster" come to mind? :) or anhydrous $\ce{CuSO4}$ by using low-temperature molten solutions of... something or other? Not sure what!

And what a delightful crystal that would be to have! It would be way out of the ordinary despite being so very... ordinary?... in one sense in its composition.


Update: Apparently, clear crystals of anhydrous $\ce{CuSO4}$ do indeed exist, and are easier to create than I thought. You just add concentrated sulfuric acid [a] to ordinary copper sulphate pentahydrate solution and let it evaporate slowly. Since $\ce{H2SO4}$ is one of the most aggressive drying agents in existence, my guess is that the $\ce{CuSO4}$ just can't compete for the remaining water molecules and is forced to crystallize without hydration.

[a] Please, never ever attempt to use concentrated $\ce{H2SO4}$anywhere except in a real lab with protections and procedures fully in place. This is a compound that wants water so badly that it willy synthesize water right out of the hydrogen and oxygen bonded into proteins and carbohydrates, leaving only charred carbon. Ironically, the actual acidity of concentrated (vs hydrated) $\ce{H2SO4}$ is quite moderate, comparable to that of vinegar.


Addenda:

-- Another common-exotic crystal may be lithium fluoride, $\ce{LiF}$, although the accuracy of that seems to depend on who you read. Unique how, you say? Even though lithium was an early result of the Big Bang, lithium and fluorine are both consumed rapidly by large hot stars, especially fluorine. Their existence on earth thus appears to depend on processes such as proton irradiation and possibly even intensive neutrino irradiation tht only occur during the explosion of a supernova star, rather than being inherited from before the cataclysmic event. Pretty much all of the elements (except $\ce{H}$) here on Earth are star-forged of course, but the elements in $\ce{LiF}$ crystals thus may be remnants of the supernova event itself. That would... cool!)

-- And speaking of unique, wearable crystals: I've always wondered if there's a market out there for diamond crystals synthesized using carbon that was captured as carbon dioxide during the cremation of a loved one. If there is, feel free -- that is definitely not my bailiwick!

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    $\begingroup$ As a possibly useful addition: structures for chalcanthite, the pentahydrate, and chalcocyanite, the anhydrous copper sulphate. Also, cremation diamonds. (Though, made from ash rather than carbon dioxide.) $\endgroup$
    – Aesin
    May 29, 2012 at 18:00
  • $\begingroup$ Aesin, that cremation diamonds link is... remarkable! I had that idea longer ago than I care to admit. Clearly a missed opportunity. And thanks for all the structures, wow. $\endgroup$ May 29, 2012 at 22:37
  • $\begingroup$ Yeah, I was looking for how it forms a crystal. It's OK, I know about conc $\ce{H2SO4}$ (though others may not). Of course, in concentrated form it only goes till $\ce{HSO4-}$, making it not that great, but the water thingy is always there. And the anhydrous crystals concept is interesting--most probably they just retain a brittle structure after dehydration. $\endgroup$ May 30, 2012 at 0:43
  • $\begingroup$ Hmm. You have a solid correct answer now -- I'm inclined just to delete this one as overly irrelevant?... $\endgroup$ May 30, 2012 at 11:22
  • $\begingroup$ @TerryBollinger I wouldn't delete it...the suggestion to dehydrate the copper (II) sulfate with sulfuric acid was interesting I thought. I have no idea what would happen, and I would try it if I still had a lab. Multiple answers usually give interesting ways to think about things. $\endgroup$ May 31, 2012 at 1:16
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I learned something in trying to answer this question since I've never quite understood what the water was doing in hydrated crystals either. The waters of hydration help to stabilize the crystalline form by holding the crystal together with hydrogen bonding. Water incorporated into the crystal allows the negative dipole of the water to partially diffuse the positive charge and the positive dipole of the water to partially diffuse the negative charge, and would help to minimize repulsive forces between ions of like charge "close" in the crystal. Speaking of $\ce{Cu(SO4)2 .5H2O}$...

Copper is surrounded by six oxygen atoms, provided by two different sulfate groups and four molecules of water. A fifth water resides elsewhere in the framework but does not bind directly to copper.....Water of crystallization is stabilized by electrostatic attractions, consequently hydrates are common for salts that contain +2 and +3 cations as well as -2 anions. 1 (Ref)

A more interesting reference that takes some looking at to get the sense of is given below. $\ce{Fe2(SO4)3.xH2O}$, with iron with a 3+ charge, forms various hydrates with x = 6, 7, 9, 10. (Disclaimer....actually this is the crystal structure for coquimbite with has an occasional Al for Fe substitution thrown in, but the principle is the same)

enter image description here

In this picture, the sulfate ions are shown as tetrahedra and the $\ce{Fe^3+}$ ions are shown as octahedra (presumably because $\ce{Fe^3+}$ ions have 6-coordination sites). The crystal is formed from "clusters of 6 sulfate tetrahedra and 3 iron octahedra which share only corners" and there are two clusters per unit cell. Each of the clusters is linked to the others through hydrogen bond provided by the incorporated water, and parallel chains are held together only through hydrogen bonds.

The individual chain segments are linked through hydrogen bonds only. The geometrical arrangement of the chains gives rise to "channels," ... which are occupied by water molecules linked to the chains by hydrogen bonds.

So, you can see how removing the waters of hydration would cause the crystalline structure to break down and result in an amorphous substance.

Further information from the paper: Coquimbite has $\ce{.9H2O}$. The crystal structure shows that, of the 3 Fe found in a cluster, one Fe is associated with 4 oxygens from sulfate groups, the second Fe is surrounded by 3 oxygens from sulfate groups and 3 water molecules, and the third Fe (or Al) is surrounded by 6 water molecules, so the octahedral representation does make sense.

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  • $\begingroup$ Hmm, interesting. I'll wait a bit and then accept. Any reason why $\ce{Fe^{3+}}$ ions are shown as octahedra? Is it because there's some coordination compound being formed? (Probably it, the "incorporated water" may be acting as a ligand on the Iron side) $\endgroup$ May 30, 2012 at 0:53
  • $\begingroup$ Could you incorporate that into your post? Thanks :) $\endgroup$ May 30, 2012 at 9:11

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