What does $n$ represent in "hydrated compound⋅nH2O"? [closed]








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On [Wikipedia](https://en.wikipedia.org/wiki/Hydrate?oldformat=true) it is written that in "hydrated compound⋅nH2O" $n$ is the number of water molecules per formula unit of the salt. I don't understand what that means and this is indicated in that I can't express it in other terms for being able to express a statement in other terms is a sign of understanding it. Can you express this in other terms? How do you imagine these n water molecules to be arranged in a formula unit?




*OrangeApple2 . n Plums means that for each orange and 2 apples, there is n plums, regardless of how oranges, apples and plums are mutually arranged in the sack.*


Now more seriously:
When there is a chemical formula like $\ce{AB2. n H2O}$, then it means there is substance $\ce{AB2}$ (ionic like $\ce{A^2+}$ + $\ce{2 B-}$, molecular like $\ce{B-A-B}$ or large scale bound with A:B ratio 1:2), which incorporates in its solid structure (crystals, microcrystalic, amorphous) molecules of water in ratio 1 $\ce{AB2}$ : $\ce{n H2O}$.


Usually, such $\ce{AB2}$ can exist as anhydrate ( being anhydrous) too, without water (like there is anhydrous citric acid and its monohydrate).


Some substances can exist in several variants with different values of $n$. E.g. [calcium chloride](https://en.wikipedia.org/wiki/Calcium_chloride) $\ce{CaCl2}$ can be anhydrous (n=0),monohydrate(1), dihydrate(2), tetrahydrate(4), hexahydrate(6).


The formula gives generally nothing more nor less than the stoichiometric substance : water ratio, without any hint about the particular structure and molecular/atomic/ionic placement. In a case of salts, some water molecules wrap cations similarly as in solution, some may be independent or attached to anions.


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