Anhydrous Copper Sulphate is a white colored salt. When water is added to Copper Sulphate, it forms a blue colored substance called Copper (II) Sulphate pentahydrate.
In the pentahydrate, copper(II) is bonded to four molecules of water and to the oxygen atoms of two sulfate anions. The fifth water is not coordinated, but links the sulfate anions via hydrogen bonding.
In the lab you can heat a sample of copper sulfate to dry it, forming anhydrous copper sulfate. You will notice the compound becoming more white in appearance as it dries. Adding water will cause the color to change back to a blue color is the compound switches back to the hydrated version.
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