Anhydrous nickel sulfate is a light yellow compound with a greenish hue. Its solutions are green, just like the solutions of the hydrated form of nickel sulfate. There is a really large difference in the speed at which the hydrated form and the anhydrous forms dissolve. The hydrated form dissolves quickly in water (minutes), while the anhydrous form takes days to dissolve completely. This sample was prepared by heating the hydrated form of nickel sulfate for half an hour at approximately 200 °C.