Colors of chemicals

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Most research into chemical structure is some form of spectroscopy. Spectroscopy is the careful measurement of the absorption 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 concept of the electromagnetic spectrum. The wavelengths correspond to energies (i.e. the energy of one photon - see Planck's constant). The visible spectrum corresponds to a wavelength range from approximately 380 nm to 760 nm.

There are also energies associated with chemical structures, and these may be described by quantum mechanics. This model says that molecular species go from one energy level to another without ever having intermediate energies.

These discontinuous level, however, in chemical structure have corresponding specific energy changes: if you want to go from level A to level B, you will always need exactly the correct amount of energy to make that jump. And that energy can come from a photon with the corresponding frequency.

So a compound will absorb photons with the energies required to make any of the jumps between energy levels within itself. Different levels of energy tend to excite different types of energy level: Infrared photons excite vibrational energy levels, Radio frequencies can excite nuclear resonance, and so on.

As an example of visible spectroscopy, 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.

The permanganate (VII) ion, 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 these ions absorb green light, and what is left of the spectrum appears violet. It 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.

1 Ions in aqueous solution
2 Salts
3 Oxidising Metals
4 Oxidising Gases

[edit] 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	VO₄^3-	None
Chromate	CrO₄ ^2-	Yellow
Dichromate	Cr₂O₇^2-	Orange
Manganese (II)	Mn²⁺	Light pink
Manganate (VII) (Permanganate)	MnO₄^-	Deep violet
Manganate (VI)	MnO₄^2-	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⁺	Royal Blue
Zinc (II)	Zn²⁺	None
Silver	Ag⁺	None

[edit] Salts

Name	Formula	Color
Copper (II) sulfate	CuSO₄	Blue
Copper (II) sulfate pentahydrate	CuSO₄ · 5H₂O	Blue
Cobalt (II) chloride	CoCl₂	Deep blue
Cobalt (II) chloride hexahydrate	CoCl₂ · 6H₂O	Deep magenta
Manganese(II) chloride tetrahydrate	MnCl₂ · 4H₂O	Pink
Copper(II) chloride dihydrate	CuCl₂ · 2H₂O	Blue-green
Nickel(II) chloride hexahydrate	NiCl₂ · 6H₂O	Green