Crystal Geometry of Anhydrous, Hydrate Forms
Date: December 2006
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?
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.
The link above is included for a more in-depth article on X-ray diffraction.
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.
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.
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Update: June 2012