What is the formula of copper sulfate

What color is the copper (II) ion?

Experiment: Reversible dehydration of copper sulfate pentahydrate

If you look at copper sulfate, you can see beautiful blue crystals. The formula is on the bottle label:

CuSO4· 5 H.20

So there is also five molar parts of water in this salt. Such a formula also has the colorless Glauber's salt, well2SO4· 10 H.2O. Since both water molecules and sulfate ions are colorless, shouldn't the blue color come from the copper ions?

Blue copper sulfate
(Photo: Sandra)

But if you carefully heat the blue crystals, water escapes and, strangely enough, you get a white salt. On the other hand, it turns blue again as soon as it comes into contact with water again (-> try). (This reaction is also used to detect water.)

What color is the copper ion now?
To clarify this, we have to take a closer look at the molecular structure of copper sulfate. The water molecules form a square-planar structure around the copper ion. This changes the color of the metal ion to blue. This structure is destroyed by heating, and the copper sulfate is again in its pure, colorless form.
This type of connection between a metal ion and water molecules is commonly called a Complex compound

(complexus (Latin) = connected). In this particular case, it's one Water- or (Latin) Aqua complex.

Four of the five water molecules are arranged in a covalent bond around the copper ion, while the fifth is bonded to the sulfate ion via hydrogen bonds. The crystal lattice is also built up from these complex ions. To take this into account, the formula of hydrous copper sulfate is written as follows:

[Cu (H2O)4]SO4· H2O

The bare copper (II) ion is therefore colorless!

And as for the Glauber's salt: This is not a complex, but a salt hydrate. That means sodium sulfate decahydrate. In the crystal of Glauber's salt, too, the water molecules are not bound, but are only loosely fixed. The bonds between water molecules and ions are comparable to those in a saline solution. However, the water molecules in the crystal lattice are strictly aligned and fixed in a highly ordered manner - but not complexly bound to the Na ion. This is the difference to the copper sulfate pentahydrate crystal.

Further texts on the subject of complex compounds

This page is part of a large website offering with further texts and experimentation instructions on Prof. Blume's educational server for chemistry.
Last revision: December 11, 2008, Dagmar Wiechoczek