The Colors of chemicals reference article from the English Wikipedia on 24-Apr-2004
(provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Colors of chemicals

Sponsor with the world's largest charity for orphans
Most research into chemical structure is some form of spectroscopy. Spectroscopy is the careful measurement of the absorbance by unknown materials by a range of electromagnetic radiation. Human sensitivity to electromagnetic radiation is limited to visible light. A chemist uses instruments to extend the detectable range, but in a broad sense, the question is 'what color is this?'.

Fundamental to physics and chemistry is the rather non-intuitive idea of the electromagnetic spectrum. The wavelengths correspond to energies (i.e. the energy of one photon). The visible light, of which we are so accustomed, is in the range of approximately 400 to 700 nm.

There are also energies associated with chemical structures; in fact, discreet energies may be associated with every facet of chemistry.

Chemical behavior is described by the model of quantum mechanics. This model says that molecular structures go from one form to another with no intermediate. There is no way to intuitively understand this. Students and teachers can almost always relate new learning to past experience, but not so in this case.

These discontinuous events, however, in chemical structure have corresponding specific energy changes. And these energy changes have a corresponding electromagnetic frequency.

So, what causes color?

The working part of hemoglobin is a heme group. The structure is such that the electrons circle around in quite large clouds. If the heme is in the form of oxyhemoglobin the clouds are a little smaller. The bluer wavelengths are absorbed and the longer red wavelengths predominate. The blood is red. If the hemoglobin is not oxygenated, the clouds are a little larger. The redder wavelengths are absorbed and the blood appears bluish.

Acid/base indicators are familiar to most students. Phenolphthalein is a large organic compound. Its structure changes depending on the acidity of the environment. In an acid environment it has no electron clouds "small enough" to absorb visible light. In a basic environment, the structure changes just a little and the electron clouds shrink just a little and the phenolphthalein turns pink.

Permanganate, MnO4-, is intensely purple/violet. Even a very dilute solution will be highly colored. Concentrated solutions appear to be completely black. The reason is that permanganate-ions absorb green light, and what is left of the spectrum appears violet. Permanganate is imagined to be a so-called charge transfer complex, which means that the electrons from the oxygen's full 2p-orbitals are temporarily "lifted" into the manganese's empty 4s- and 3d-orbitals by photons, which in that process are absorbed.

Visible light corresponds to the energy absorbed in many large organic structures. Evolution has converged on these as the most useful wavelengths for our eyes.

It is all very interesting. What causes the colors of compounds? The structure causes the color. Changes in color are a reflection of changes in structure.





Ions in aqueous solution




   

   
Name   
Formula   
Color   

 

   
Alkali metals 
M+ 
none 



   
Alkaline earth metals 
M2+ 
None



   
Scandium (III) 
Sc3+ 
None



   
Titanium (III) 
Ti3+ 
Violet



   
Titanyl    
TiO2+   
None   



   
Vanadium (II)   
V2+   
Lavender  



   
Vanadium (III)   
V3+   
Dark grey/green   



   
Vanadyl   
VO2+   
Blue  



   
Pervanadyl   
VO2+   
Yellow  



   
Metavanadate   
VO3-   
None  



   
Orthovanadate    
VO43-   
None  



   
Chromate    
CrO4 2-   
Yellow  



   
Dichromate   
Cr2O72-   
Orange  



   
Manganese (II)   
Mn2+   
Light pink  



   
Permanganate   
MnO4-   
Deep violet  



   
Manganate (VI) 
MnO42-   
Dark green  



   
Manganate (V)   
MnO4 3-  
Deep blue  



   
Iron (II)   
Fe2+   
Light green  



   
Iron (III)   
Fe3+   
Yellow/brown  



   
Cobalt (II)   
Co2+   
Light red  



   
Nickel (II)   
Ni2+   
Light green   



   
Nickel-ammonium complex   
Ni(NH3)62+   
Lavender/blue  



   
Copper (II)   
Cu 2+   
Blue  



   
Copper-ammonium complex   
Cu(NH3)42+   
Blue  



   
Zinc (II)   
Zn2+   
None  



   
Silver   
Ag+   
None  







Salts




   

   
Name   
Formula   
Color   

 

   
Copper (II) sulfate   
CuSO4   
Light green  



   
Copper (II) sulfate pentahydrate   
CuSO4 · 5H2O   
Blue  



   
Cobalt (II) chloride   
CoCl2   
Deep blue  



   
Cobalt (II) chloride hexahydrate   
CoCl2 · 6H2O   
Light red