Color of chemicals

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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.


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:

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[edit]

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]

Color Complementary
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[edit]

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.


Ions in aqueous solution[edit]

Colors of ions in aqueous solution[2]
Name Formula Color
Alkali metals M+ Colorless
Alkaline earth metals M2+ Colorless
Scandium(III) Sc3+ Colorless
Titanium(III) Ti3+   Violet
Titanium(IV) Ti4+ Colorless
Titanyl TiO2+ Colorless
Vanadium(II) V2+   Lavender
Vanadium(III) V3+   Dark grey-green
Vanadyl(IV) VO2+   Blue
Vanadate(IV) (vanadite) V
Vanadyl(V) (pervanadyl) VO+
Metavanadate VO
Orthovanadate VO3−
Chromium(II) Cr2+   Bright blue
Chromium(III) Cr3+   Blue-green-grey
Chromium(III) Sulphate CrSO4+   Dark Green
Chromium(III) Hydroxide Cr(OH)63-   yellowish
Chromate CrO2−
Dichromate Cr
Manganese(II) Mn2+   Pale pink
Manganese(III) Mn3+   Deep pink/red
Manganate(V) MnO3−
  Deep blue
Manganate(VI) MnO2−
  Dark green
Manganate(VII) (permanganate) MnO
  Deep purple
Iron(II) Fe2+   Very pale green
Iron(III) Fe3+   Very pale violet/brown
Iron(III) tetrachloro complex FeCl
Cobalt(II) Co2+   Pink
Cobalt(III) ammine complex Co(NH
Nickel(II) Ni2+   Light green
Nickel(II) ammine complex Ni(NH
Copper(I) ammine complex Cu(NH
Copper(II) Cu2+   Blue
Copper(II) ammine complex Cu(NH
Copper(II) tetrachloro complex CuCl2−
Zinc(II) Zn2+ Colorless
Silver(I) Ag+ Colorless
Silver(III) (in conc. HNO3) Ag3+   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.


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
Chromium (III) Hydroxide Cr(OH)3 Green
Copper(II) sulfate (anhydrous) CuSO4 White Anhydrous copper(II) sulfate
Copper(II) sulfate pentahydrate CuSO4·5H2O Blue Large crystals of copper sulfate
Copper(II) benzoate Cu(C7H5O2)2 Blue Powdered copper benzoate
Cobalt(II) chloride CoCl2 Deep blue Cobalt(II) chloride
Cobalt(II) chloride hexahydrate CoCl2·6H2O Deep magenta Cobalt(II) chloride hexahydrate
Manganese(II) chloride tetrahydrate MnCl2·4H2O Pink Manganese(II) chloride tetrahydrate
Copper(II) chloride dihydrate CuCl2·2H2O Blue-green copper(II) chloride dihydrate
Nickel(II) chloride hexahydrate NiCl2·6H2O Green Nickel(II) chloride hexahydrate
Lead(II) iodide PbI2 Yellow Lead(II) iodide

Ions in flame[edit]

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
Copper Cu   Blue/Green(Often with white flashes)
Lead Pb   Grey/White


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[edit]

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