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Name:  Louise
Status: other
Grade: 6-8
Country: N/A
Date: 10/12/2005

Would you please explain in simple terms (comprehendible for a 12 year old) why copper (II) sulphate is blue. I do not understand this because neither copper or sulphur is blue.

Light reflects off of things, but some colors reflect more than others. Which colors reflect the best has to do with the material. We usually just memorize this -- oranges are orange, apples are red. But the "why" question is not all that easy to answer. It has to do with the atoms that make up the material, and how they are arranged among themselves. Copper is indeed red-ish by itself, and sulfur is yellow, by itself. But when combined, their atoms work together to form an overall compound, copper sulfate. This compound is different from the atoms that formed it. It is blue. It does not conduct electricity very well (copper does....), it cannot be pulled into a wire (like copper), it cannot be mixed with charcoal to make gun powder (sulfur....).

The electrons which circle the copper atom interact with the light in a way to make copper color. Same with sulfur. But when the compound is formed, then the electrons do not circle just one single copper, but work their way around the copper, the sulfur, some oxygen atoms etc etc. The electrons see a bigger picture, and interact with incoming light in a different way. We see this as a color change.

You can actually watch a chemical reaction take place -- mix two things together, each of which has no color, and watch some colored liquid form.

Steve Ross


You already have part of the answer. You have noted that neither copper metal (red-brown) nor sulfur powder (yellow) is blue. You have also written the name copper(II) sulfate which indicates that the copper is in a cationic form of Cu2+ (the copper metal has lost two electrons), and the sulfur is in an anion complex of sulfate (SO4 2-). Thus, neither the copper nor the sulfur is in its atomic form.

Further investigations reveal that sulfate containing compounds (sulfuric acid, iron(II) sulfate, sodium sulfate, etc.) are not blue. Whereas many Cu2+ containing compounds (copper(II) nitrate, copper(II) hydroxide) especially if they are dissolved in water are blue. (Notable exceptions are those Cu2+ containing compounds that do not dissolve in water, such as CuO are not blue.)

Thus, it would seem that the combination of Cu2+ and water makes for a blue color. As to how this particular interaction provides color, I will leave for a later discussion.

Greg (Roberto Gregorius)

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