Answer to Question #54017 in Inorganic Chemistry for Shagh fazelian

Question

Hello
Is water crystallization of salts ( salt hydrates ) a physical or a chemical change?
For example:CuSO4•5H2O - copper(II) sulfate pentahydrat
if you know any sources about this subject please introduce it to me
Thank you

Answer 1

Since new compound with exact formula and mass is formed, this is chemical change. For example, CuSO4•5H2O is not a physical mixture of water and CuSO4 (there are no two phases), but it is new homogeneous phase formed upon crystallization. In addition, it is possible to obtain several different compounds from the same solution upon tuning of the temperature. For instance, there are three hydrates which can be isolated from the FeSO4 solution: FeSO4×7H2O, FeSO4×4H2O and FeSO4×H2O. Each of them has definite structure, one phase and unique content.
Physical change occurs, for instance, when a melt of two metals is cooled down which results in crystallization of one of them without including the second metal to this phase. Since there are no new species formed, this can be considered as physical change.
Some sources on physical and chemical changes:
https://schools.smcps.org/gkes/images/Changing_Matter-_Understanding_Physical_and_Chemical_Changes.pdf
About crystallization:
https://archive.org/stream/Crystallization4thEdition/Crystallization4ed2001-Mullin#page/n3/mode/2up