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I've come across two use cases of the word "hydration", presented to me as distinct topics:

  1. Solvation is the process of reorganizing solvent and solute molecules into solvation complexes and involves bond formation, hydrogen bonding, and van der Waals forces. Solvation of a solute by water is called hydration. And, the solute ion/molecule around which this hydration shell is formed is said to be hydrated.

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  1. Water of crystallization are the molecules of water which remain trapped within the lattice of a substance. The salts containing water of crystallisation are said to be hydrated.

Is it safe to assume that the two instances of the word refer to the same phenomenon?

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    $\begingroup$ see chemistry.stackexchange.com/questions/544/… $\endgroup$
    – Andrew
    Commented Feb 16, 2023 at 16:41
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    $\begingroup$ What is the source of the second illustration, and what actually is depicted? These are channels (the host structure, depicted with the red spheres) loaded with guest compounds as loading (blue wire models). The guest's position/location in the host, depending on the supramolecular interactions, sometimes is statistical, sometimes random. Yet the loading/discharge of guests into such a host is much more similar to adsorption/desorption (e.g. in zeolithes, MOFs), than crystallization from a solution/dissolution of a solid (e.g., formation of $\ce{CuSO4\cdot 5 H2O}$). $\endgroup$
    – Buttonwood
    Commented Feb 17, 2023 at 8:06

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Water of crystallization can he inherited from the hydration sphere, especially around the cation, but it is also possible to have water molecules incorporated into other parts of the crystal structure. Hydrogen bonding from the hydrogen atoms in the water molecules to oxygen atoms in oxyanions or other water molecules is one such bonding mechanism.

Copper sulfate pentahydrate, $\ce{CuSO4 * 5 H2O}$, offers a relatively well-known example. Ruggiero et al. [1] gives an experimentally determined structure in which the copper atoms are coordinated to four of the water molecules as they would be in solution, but the fifth water molecule in the formula unit is hydrogen-bonded as shown in the illustration below (from 1; teal lines added to show the hydrogen bonds).

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Reference

  1. Ruggiero, M.; Erba, A.; Orlando, R.; Korter, T. "Origins of Contrasting Copper Coordination Geometries in Crystalline Copper Sulfate Pentahydrate." Phys. Chem. Chem. Phys. 2015, 17, 31023-31029 doi: 10.1039/C5CP05554G.
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