What determines the colors which a substance will reflect? There must be some relationship between the chemical structure of a molecule and the
color of that substance. Can someone give a simple explanation of this?
It's exactly the chemical structure of the molecule that determines the
color of the solution. In organic molecules, it's usually the conjugation
of double bonds (single bond-double bond-single-double, etc) that accounts
for color. These bonds absorb certain wavelengths of white light thereby
transmitting others thus giving off a color. For inorganics (the reason
why copper solutions are blue in water), its the bonds again that absorb
certain wavelengths and transmit others. The copper-water bond absorbs
light in the 580-600 nanometer range (yellow-orange) so it transmits
blue light. A yellow solution then absorbs blue light(450 nanometers).
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Update: June 2012