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 manganate (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.
| Contents |
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 |
| Manganate (VII) (Permanganate) | MnO4- | Deep violet |
| Manganate (VI) | MnO42- | Dark green |
| Manganate (V) | MnO43- | 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 |
| Copper(II) chloride dihydrate | CuCl2 · 2H2O | Green |
Oxidising Metals
| Name | Formula | Color |
|---|---|---|
| Potassium | K | Lilac/Purple |
| Sodium | Na | Orange/Red |
Oxidising Gasses
| Name | Formula | Color |
|---|---|---|
| Hydrogen | H2 | Purple |