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. This spectral perspective was first noted in atomic spectroscopy.

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 (lycopene is responsible for the color of tomatoes). Charge-transfer complexes tend to have very intense colors for different reasons.

Examples

Ions in aqueous solution

Colors of ions in aqueous solution^[2]

Name	Formula	Color
Alkali metals	M^+	Colorless
Alkaline earth metals	M^2+	Colorless
Scandium(III)	Sc^3+	Colorless
Titanium(III)	Ti^3+		Violet
Titanium(IV)	Ti^4+	Colorless
Titanyl	TiO^2+	Colorless
Vanadium(II)	V^2+		Lavender
Vanadium(III)	V^3+		Dark grey-green
Vanadyl(IV)	VO^2+		Blue
Vanadate(IV) (vanadite)	V 4O2− 9		Brown
Vanadyl(V) (pervanadyl)	VO+ 2		Yellow
Metavanadate	VO− 3	Colorless
Orthovanadate	VO3− 4	Colorless
Chromium(III)	Cr^3+		Blue-green-grey
Chromate	CrO2− 4		Yellow
Dichromate	Cr 2O2− 7		Orange
Manganese(II)	Mn^2+		Pale pink
Manganate(V)	MnO3− 4		Deep blue
Manganate(VI)	MnO2− 4		Dark green
Manganate(VII) (permanganate)	MnO− 4		Deep purple
Iron(II)	Fe^2+		Very pale green
Iron(III)	Fe^3+		Very pale violet/brown
Iron(III) tetrachloro complex	FeCl− 4		Yellow/brown
Cobalt(II)	Co^2+		Pink
Cobalt(III) ammine complex	Co(NH 3)3+ 6		Yellow/orange
Nickel(II)	Ni^2+		Light green
Nickel(II) ammine complex	Ni(NH 3)2+ 6		Lavender/blue
Copper(I) ammine complex	Cu(NH 3)+ 2	Colorless
Copper(II)	Cu^2+		Blue
Copper(II) ammine complex	Cu(NH 3)2+ 4		Indigo-blue
Copper(II) tetrachloro complex	CuCl2− 4		Green
Zinc(II)	Zn^2+	Colorless
Silver(I)	Ag^+	Colorless
Silver(III) (in conc. HNO3)	Ag^3+		Dark brown

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 is used as a moisture indicator in silica gel. *Zinc oxide is white, but at higher temperatures becomes yellow, returning to white as it cools.

Colors of various salts

Name	Formula	Color	Picture
Copper(II) sulfate (anhydrous)	CuSO4	White
Copper(II) sulfate pentahydrate	CuSO4·5H2O	Blue
Copper(II) benzoate	Cu(C7H5O2)2	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

Colors of alkali metal and alkaline earth metal ions in flame^[3]

Name	Formula	Color
Lithium	Li		Red
Sodium	Na		Yellow/orange
Magnesium	Mg		Brilliant white
Potassium	K		Lilac/violet
Calcium	Ca		Brick red
Rubidium	Rb		Pink/red
Strontium	Sr		Red
Caesium	Cs		Light blue
Barium	Ba		Green/yellow

Gases

Colors of various gases

Name	Formula	Color
Hydrogen	H2	colorless
Oxygen	O2	colorless
Fluorine	F2		very pale yellow/brown
Chlorine	Cl2		greenish yellow
Bromine	Br2		red/brown
Iodine	I2		dark purple
Chlorine dioxide	ClO2		intense yellow
Dichlorine monoxide	Cl2O		brown/yellow
Nitrogen dioxide	NO2		dark brown
Dinitrogen tetroxide	N2O4	colorless
Trifluoronitrosomethane	CF3NO		deep blue
Diazomethane	CH2N2		yellow

Bead tests

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 fuses and the color of the resulting bead observed.

Colors exhibited by metals in the bead test

Metal[4]	Oxidizing flame	Reducing flame
Aluminium	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)
Gold	golden (hot), silver (cold)	red (hot and cold)
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), violet (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. http://www.sanjuan.edu/webpages/dkrenecki/files/ColorsofSubstances2.pdf
3. Flame Tests at chemguide.co.uk
4. CRC Handbook of Chemistry and Physics. CRC Press. 1985. ISBN 0-8493-0466-0.