Crystal Geometry of Anhydrous, Hydrate Forms

 
name         Devon
status       other
grade        other
location     MI

Question -   Does the crystal geometry of the hygroscopic
compound change between its anhydrous and hydrate forms? If so, how
could one tell how they would differ?
Devon,

The crystal form of an anhydrous form and a hydrated form must 
change, because the crystal lattice must accommodate water 
molecules.  The reason that hygroscopic (thank you for not typing 
hydroscopic;) molecules pick up water is to lower the crystal 
energy.  Typically one tells the difference by using X-ray diffraction.

http://en.wikipedia.org/wiki/X-ray_diffraction

The link above is included for a more in-depth article on X-ray diffraction.

Matt Voss
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Yes, it changes. You cannot predict how very easily,
but you can look up the crystal structures the two forms have
in scientific reference literature.

Whether you can see the transformation by eye is another matter
which I have no experience with.  It might not be real clear.

Jim Swenson
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The crystal structure of an anhydrous salt and its (one or more) 
hydrates are not related in a simple form. One factor is how water 
molecules can "pack" within the crystal structure, but just how 
this occurs cannot be determined by any simple model that I am aware of.

Vince Calder
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