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I mean looking at most of the sulphate salts (there are so many but just to name some Epsom salt, blue vitirol, gypsum...) they have some amount of water crystallized within them (Epsom salt is MgSO4.7H2O, CuSO4.5H2O is blue vitirol and so on) so why is it like that? Is there any reason why water is crystallized in almost all metal sulphates

It isn't (maybe) related to hydration energy... nor (maybe) related to the size of the sulphate ion.

Oscar Lanzi
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Sulfates commonly do have water of hydration, but so do many other salts such as chlorides. For instance, iron chlorides used in laboratory experiments at my job are commonly bought as $\ce{FeCl2•4H2O}$ and $\ce{FeCl3•6H2O}$.

One difference between sulfates and some other salts is the oxygen in the sulfate ion can form relatively strong hydrogen bonds with the protic hydrogen in water, enabling incorporation of additional water molecules into the crystal that aren't coordinated with the metal ion. Thus for instance magnesium ion commonly coordinates with six water molecules, but the sulfate salt commonly incorporates a seventh water molecules in its formula unit (Epsom salt) and can even have eleven at lower temperature.

Oscar Lanzi
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