If there are two salts in a solution, evaporating the water slowly to boil the water away, two kinds of crystals are formed. Why the each of the crystals is not contaminated by the other salt,like staying in the "core" of the crystal?

they won`t fit into the structure, it`s a bit like having Duplo bricks and lego all mixed up in a bag, only lego will fit to other lego and so on.

Is it very pure?

I`m not quite sure I understand your questions context?

 

many crystals are Very pure indeed, however under certain circumstances (Heat and Pressure mainly) Impurities can be "Added" to a crystal, take Aluminium Oxide crystals, they can be Emeralds or Rubies, they`re both Alu Oxide, but with different impurities , giving them their unique color

most crystals also contain H2O, so even "pure" CuSO4 is not really.

most crystals also contain H2O, so even "pure" CuSO4 is not really.

 

what do you mean by "Most"?

That's why we write it out as CuSO4*xH2O.

most crystals also contain H2O, so even "pure" CuSO4 is not really.

I would have to disagree with that statement. Replace "most" with "some".

 

It is also worth noting that CuSO4 does occur in nature without any waters of hydration (chalcocyanite).

Other hydration states also exist:

 

CuSO4.H20 Poitevinite

CuSO4.3H2O Bonattite

CuSO4.5H20 Chalcanthite

CuSO4.7H2O Boothite (a very rare mineral that I have recently written a paper on)

 

Each of these species have different crystal structures and definite stoichiometric amounts of water, which means the water can't really be called an impurity.