The water of crystallization is the total weight of water in a substance at a given temperature and is mostly present in a
definite ratio.
On heating, iron(II) sulfate first loses its water of crystallization and the original green crystals are converted into a brown-colored anhydrous solid.
FeSO4.7H2O is the heptahydrate in solution (water as solvent) transforms to both heptahydrate and tetrahydrate when the temperature reaches 56.6∘C.