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The formula for borax is written as $\ce{Na2B4O7.10H2O}$ and also as $\ce{Na2[B4O5(OH)4].8H2O}$.
From the structure, it obvious from where the 2nd formula comes. But the question I have is, what is the water of crystallisation in Borax, 8 or 10?

More specifically, I was to know what's the proper defintion of water of crystallisation? If it is just the number of water molecules present in the crystal structure, not directly bonded to the molecule (Which is what I thought) then the answer should be 8.
But one my teachers said it should be 10, as written in the formula, while the other who used to say 8 couldn't give an explanation to following

"Borax, when heated loses all 10 water molecules. When dissolved in water, all 10 water molecules are lost*. So even though they're bonded to the boron, the 2 water molecules behave the same way as the other 8, chemically"
*Just to mention, the full compound breaks up anyways, so I'm not so convinced with this argument

Another compound which came up in my mind during this was $\ce{CuSO4.5H2O}$, The structure of which is Structure of CuSO4.5H2O

What would be the water of crystalisation in Copper Sulphate? I'm sure there will be lot more compounds having $\ce{H2O}$ molecules bonded like this. What would be the "Water of crystallisation" in them and what's the proper definition of it?

Unlike Borax, Copper Sulphate doesn't lose all water molecules at once on heating, just opposite of the argument given above for Borax. Heating reaction of Copper Sulphate

Edit: I found this on wikipedia. Image from https://en.wikipedia.org/wiki/Water_of_crystallization

So, according to this the water of crystalisation in $\ce{CuSO4.5H2O}$ should be 1, but according to any website which speaks on this, the water of crystalisation is 5. So what is the actual definition? Or it is just a broad term like mentioned in a comment?

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  • $\begingroup$ Perhaps "water of crystallization" is too broad of a term. The question clarifies the actual crystal water nicely. Perhaps you could write Na2B4O7.2H2O.8H2O and CuSO4.H2O.4H2O, suggesting two kinds of crystalline water: tight and looser. $\endgroup$ Jan 2 at 17:04
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    $\begingroup$ Does this answer your question? Structure of Borax $\endgroup$ Jan 2 at 17:06
  • $\begingroup$ And chemistry.stackexchange.com/questions/171016/… with the links therein $\endgroup$ Jan 2 at 17:07
  • $\begingroup$ @NilayGhosh I was mostly concerned with "Water of crystallization" and what it actually implied. I saw the other question related to borax, and some others, they didn't talk about water of crystallisation directly. I also found some detail on wikipedia, adding it $\endgroup$ Jan 3 at 1:58
  • $\begingroup$ A quoted text is better to be presented as a text ( with the quote character > ) than as a screenshot. $\endgroup$
    – Poutnik
    Jan 3 at 6:08

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