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 Planck-Einstein relation

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, Ligand Field Theory and Crystal 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  Yellow
424–491  Blue  Orange
491–570  Green  Red
570–585  Yellow  Violet
585–647  Orange  Blue
647–700  Red  Green

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

Colors of metallic ions
Name Formula Color
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
Vanadium(IV) (vanadite) V
4
O2−
9
 Brown
Vanadium(V) (pervanadyl) VO+
2
 Yellow
Metavanadate VO
3
Colorless
Orthovanadate VO3−
4
Colorless
Chromium(II) Cr2+  Bright blue
Chromium(III) Cr3+  Blue-green-grey
Chromium(III) sulfate CrSO4+  Dark Green
Chromium(III) hydroxide Cr(OH)63−  yellowish
Monochromate CrO2−
4
 Yellow
Dichromate Cr
2
O2−
7
 Orange
Manganese(II) Mn2+  Pale pink
Manganese(III) Mn3+  Crimson
Manganate(V) MnO3−
4
 Deep blue
Manganate(VI) MnO2−
4
 Dark green
Manganate(VII) (permanganate) MnO
4
 Deep purple
Iron(II) sulfate Fe2+  Very pale green
Iron(III) oxide-hydroxide FeO(OH)  Dark brown
Iron(III) tetrachloro complex FeCl
4
 Yellow/brown
Cobalt(II) fluoride Co2+  Pink
Cobalt(III) ammine complex Co(NH
3
)3+
6
 Yellow/orange
Nickel(II) Ni2+  Light green
Nickel(II) ammine complex Ni(NH
3
)2+
6
 Lavender/blue
Copper(I) ammine complex Cu(NH
3
)+
2
Colorless
Copper(II) Cu2+  Blue
Copper(II) ammine complex Cu(NH
3
)2+
4
 Indigo-blue
Copper(II) tetrachloro complex CuCl2−
4
 Green
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.

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 of the corresponding salts
Color Picture
Chromium(III) hydroxide Cr(OH)3 Green
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[2]
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

Gases

Colors of various gases
Name Formula Color
Hydrogen H2 colorless
Oxygen O2 colorless
Ozone O3  very pale blue
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
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[3]Oxidizing flameReducing flame
Aluminumcolorless (hot and cold), opaquecolorless, opaque
Antimonycolorless, yellow or brown (hot)gray and opaque
Bariumcolorless
Bismuthcolorless, yellow or brownish (hot)gray and opaque
Cadmiumcolorlessgray and opaque
Calciumcolorless
Ceriumred (hot)colorless (hot and cold)
ChromiumDark yellow (hot), green (cold)green (hot and cold)
Cobaltblue (hot and cold)blue (hot and cold)
Coppergreen (hot), blue (cold)red, opaque (cold), colorless (hot)
Goldgolden (hot), silver (cold)red (hot and cold)
Ironyellow or brownish red (hot and cold)green (hot and cold)
Leadcolorless, yellow or brownish (hot)gray and opaque
Magnesiumcolorless
Manganeseviolet (hot and cold)colorless (hot and cold)
Molybdenumcolorlessyellow or brown (hot)
Nickelbrown, red (cold)gray and opaque (cold)
Siliconcolorless (hot and cold), opaquecolorless, opaque
Silvercolorlessgray and opaque
Strontiumcolorless
Tincolorless (hot and cold), opaquecolorless, opaque
Titaniumcolorlessyellow (hot), violet (cold)
Tungstencolorlessbrown
UraniumYellow or brownish (hot)green
Vanadiumcolorlessgreen

References

  1. http://www.sapdesignguild.org/resources/glossary_color/index1.html
  2. Flame Tests at chemguide.co.uk
  3. CRC Handbook of Chemistry and Physics. CRC Press. 1985. ISBN 0-8493-0466-0.
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