水合硫酸銅(II)是否一定是五水合硫酸銅(II)?

五水合硫酸銅(II) CuSO4.5H2O is commonly exist in natural, but it is not the only hydrated form!
The blue colour of CuSO4.5H2O is due to the copper is surrounded by 4 water, i.e. [Cu(H2O)4]2+ . Therefore, Cu atom is surrounded by 6 oxygen atom.
the remaining water provides hydrogen bonding to link [Cu(H2O)4]2+ (SO4)2- molecules together, thus give a crystalline structure.

When you heat this CuSO4.5H2O, the 4 water surrounding Cu atom would come out first, then you can get a white crystalline CuSO4.H2O powder (e.g. like table salt). This contains one water only.

When you further heat it, you will not see the crystalline structure, and you will only see white amorphous powder (e.g. not shiny, like starch), and this is anhydrous CuSO4.