Copper Sulfate Color
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.
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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Update: June 2012