Color of chemicals

The color of chemicals is a physical property of chemicals that in most cases comes from the excitation of electrons due to an absorption of energy performed by the chemical. What is seen by the eye is not the color absorbed, but the complementary color from the removal of the absorbed wavelengths.

The study of chemical structure by means of energy absorption and release is generally referred to as spectroscopy.

Theory

The UV-vis spectrum for a compound that appears orange in Dimethylformamide

All atoms and molecules are capable of absorbing and releasing energy in the form of photons, accompanied by a change of quantum state. The amount of energy absorbed or released is the difference between the energies of the two quantum states. There are various types of quantum state, including, for example, the rotational and vibrational states of a molecule. However the release of energy visible to the human eye, commonly referred to as visible light, spans the wavelengths approximately 380 nm to 760 nm, depending on the individual, and photons in this range usually accompany a change in atomic or molecular orbital quantum state. The perception of light is governed by three types of color receptors in the eye, which are sensitive to different ranges of wavelength within this band.

The relationship between energy and wavelength is determined by the equation:

$E = hf = \frac{hc}{\lambda} \,\!$

where E is the energy of the quantum (photon), f is the frequency of the light wave, h is Planck's constant, λ is the wavelength and c is the speed of light.

The relationships between the energies of the various quantum states are treated by atomic orbital, molecular orbital, and Ligand Field Theory. If photons of a particular wavelength are absorbed by matter, then when we observe light reflected from or transmitted through that matter, what we see is the complementary color, made up of the other visible wavelengths remaining. For example beta-carotene has maximum absorption at 454 nm (blue light), consequently what visible light remains appears orange.

Colors by wavelength

Below is a rough table of wavelengths, colors and complementary colors. This utilizes the scientific CMY and RGB color wheels rather than the traditional RYB color wheel.^[1]

Wavelength (nm)	Color	Complementary Color
400-424	Violet	Green-yellow
424-491	Blue	Yellow
491-570	Green	Violet
570-585	Yellow	Blue
585-647	Orange	Cyan-Blue
647-700	Red	Cyan

This can only be used as a very rough guide, for instance if a narrow range of wavelengths within the band 647-700 is absorbed, then the blue and green receptors will be fully stimulated, making cyan, and the red receptor will be partially stimulated, diluting the cyan to a greyish hue.

By category

The vast majority of simple inorganic (e.g. sodium chloride) and organic compounds (e.g. ethanol) are colorless. Transition metal compounds are often colored because of transitions of electrons between d-orbitals of different energy. (see Transition metal#Coloured compounds). Organic compounds tend to be colored when there is extensive conjugation, causing the energy gap between the HOMO and LUMO to decrease, bringing the absorption band from the UV to the visible region. Similarly, color is due to the energy absorbed by the compound, when an electron transitions from the HOMO to the LUMO. Lycopene is a classic example of a compound with extensive conjugation (11 conjugated double bonds), giving rise to an intense red color. Charge-transfer complexes tend to have very intense colors for different reasons.

Examples

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
Chromium(III)	Cr3+	Blue-green
Chromate	CrO4 2-	Colorless or Yellow(sometimes)
Dichromate	Cr2O72-	Orange
Manganese(II)	Mn2+	Colourless
Manganate(VII) (Permanganate)	MnO4-	Deep violet
Manganate(VI)	MnO42-	Dark green
Manganate(V)	MnO43-	Deep blue
Iron(II)	Fe2+	Light blue
Iron(III)	Fe3+	Yellow/brown
Cobalt(II)	Co2+	Pink
Cobalt-ammonium complex	Co(NH3)63+	Yellow/orange
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
Tetrachloro-copper complex	CuCl42-	Yellow/green
Zinc(II)	Zn2+	Bluish-white
Silver	Ag+	None

It is important to note, however, that elemental colors will vary depending on what they are complexed with, often as well as their chemical state. An example with vanadium(III); VCl₃ has a distinctive reddish hue, whilst V₂O₃ appears black.

Salts

Predicting the color of a compound can be extremely complicated. Some examples include: Cobalt chloride is pink or blue depending on the state of hydration (blue dry, pink with water) so it's used as a moisture indicator in silica gel. Zinc Oxide is white, but at higher temperatures becomes yellow, returning to white as it cools.

Name	Formula	Color	Picture
Copper(II) sulfate	CuSO4	White
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
Lead(II) iodide	PbI2	Yellow

Ions in Flame

Flame Tests on cations for Alkali, Alkali Earth Metals, and Hydrogen (see atomic spectroscopy) (see also Flame test)

Metals

Name	Formula	Color
Potassium	K	Lilac/Purple
Sodium	Na	Yellow
Lithium	Li	Red
Caesium	Cs	Blue
Calcium	Ca	Red/Orange
Strontium	Sr	Red
Barium	Ba	Green/Yellow

Gases

Name	Formula	Color
Hydrogen	H2	colorless

Bead tests

Main article: Bead test

A variety of colors, often similar to the colors found in a flame test, are produced in a bead test, which is a qualitative test for determining metals. A platinum loop is moistened and dipped in a fine powder of the substance in question and borax. The loop with the adhered powders is then heated in a flame until it fusses and the color of the resulting bead observed.

Metal[2]	Oxidising flame	Reducing flame
Aluminum	colorless (hot and cold), opaque	colorless, opaque
Antimony	colorless, yellow or brown (hot)	gray and opaque
Barium	colorless
Bismuth	colorless, yellow or brownish (hot)	gray and opaque
Cadmium	colorless	gray and opaque
Calcium	colorless
Cerium	red (hot)	colorless (hot and cold)
Chromium	Dark yellow (hot), green (cold)	green (hot and cold)
Cobalt	blue (hot and cold)	blue (hot and cold)
Copper	green (hot), blue (cold)	red, opaque (cold), colorless (hot)
Iron	yellow or brownish red (hot and cold)	green (hot and cold)
Lead	colorless, yellow or brownish (hot)	gray and opaque
Magnesium	colorless
Manganese	violet (hot and cold)	colorless (hot and cold)
Molybdenum	colorless	yellow or brown (hot)
Nickel	brown, red (cold)	gray and opaque (cold)
Silicon	colorless (hot and cold), opaque	colorless, opaque
Silver	colorless	gray and opaque
Strontium	colorless
Tin	colorless (hot and cold), opaque	colorless, opaque
Titanium	colorless	yellow (hot), biolet (cold)
Tungsten	colorless	brown
Uranium	Yellow or brownish (hot)	green
Vanadium	colorless	green

References

1. ^ http://www.sapdesignguild.org/resources/glossary_color/index1.html
2. ^ CRC Handbook of Chemistry and Physics. CRC Press. 1985. ISBN 0849304660.