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I am trying to understand adsorption and hydration states better, and I was thinking about the different hydration states of calcium sulfate - anhydrous ($\ce{CaSO4}$), hemihydrate ($\ce{CaSO4*1/2H2O}$), or dihydrate ($\ce{CaSO4*2H2O}$).

I would assume that $\ce{CaSO4}$ and $\ce{CaSO4*1/2H2O}$ would adsorb more water than $\ce{CaSO4*2H2O}$, and I am suspecting that $\ce{CaSO4*2H2O}$ barely adsorbs any water at all since it is "saturated" in a way. But I cannot find any information on this. In fact, I can't seem to find any sources mentioning adsorption in the context of hydration, which is surprising to me - I would intuitively assume that these phenomenons are closely related.

I would therefore appreciate an explanation of the relation of adsorption and hydration. Basically, does a lower hydration state generally imply stronger adsorption? Is adsorption in fact the dominant process by which a chemical changes its hydration state? And how does desorption work for hydrated chemicals?

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  • $\begingroup$ The three are different compounds with different crystal structures, you should not assume such relations. Of course given time, they should turn into one another, but how long that takes, how much energy each step releases? $\endgroup$
    – Karl
    Commented Aug 26, 2020 at 21:51
  • $\begingroup$ @Karl, that is why I am asking the question! I want to know whether there is a general rule that fits my assumption, but it is entirely possible that no such rule exists. However, bear in mind that the crystal structure breaks down at the surface, which is where adsorption occurs. $\endgroup$
    – PoorYorick
    Commented Aug 27, 2020 at 8:30
  • $\begingroup$ Adsorption is a surface process, and, by its definition, not a chemical reaction. Hydration however is. $\endgroup$
    – Karl
    Commented Aug 27, 2020 at 19:31

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Surprisingly enough, hemihydrate reacts quickly with water to produce dehydrate (gypsum). It is due to the fact that hemihydrate is produced by heating gypsum to a high temperature. This heating produces a structure with plenty of holes or voids corresponding to the previous positions of the water molecules. Water molecules can easily fill these holes if hemihydrate is mixed with water. Anhydrous calcium sulfate is a mineral (anhydrite) that does not react easily with water. The structure of anhydrous calcium sulfate is compact and does not contain any holes between its atoms to include easily water molecules.

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