Copper Sulfate and Color

Name: Josh
Status: student
Grade: 9-12
Location: Australia

Question: Why does Copper Sulfate turn white once it has lost all
its 5 waters, and then blue when rehydrated? I did an experiment
recently that involved evaporating water from 55 ml of Copper
Sulfate Solution so that solid copper sulfate remained. When all the
water has been evaporated, a blue "powder" was left (or were they
Copper Sulfate crystals?). However, the Copper Sulfate "powder" was
heated a little too long, and they turned white. Why does the Copper
Sulfate turn white when it loses its waters? Why does it turn blue
when hydrated?
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Josh,
 
Light and matter interaction requires that light be able to be absorbed 
and then transmitted by the molecule(s) involved. If the molecule does 
not partially absorb light in the visible range of the spectrum, then 
the molecule will appear white (transmitting all visible light). When 
copper sulfate anhydrate interacts with light it transmits all the 
visible light without absorbing any of it, as such it appears white. 
The presence of water in copper sulfate pentahydrate allows for some 
mechanism for part of the light to be absorbed and the blue part of 
the light to be transmitted, making the hydrated form look blue.

Greg (Roberto Gregorius)
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