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.
The study of chemical structure by means of energy absorption and release is generally referred to as spectroscopy.
Theory
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 the Planck 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 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.
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. Wavelength(nm) Color Complementarycolor 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 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#Colored 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 Magnesium(II) Mg2+ colorless Scandium(III) Sc3+ silver Titanium(III) Ti3+ purple Titanium(IV) Ti4+ silver Titanyl TiO2+ colorless Vanadium(II) V2+ light purple Vanadium(III) V3+ dark grey-green Vanadyl(IV) VO2+ blue Vanadium(IV) (vanadite) brown Vanadium(V) (pervanadyl) yellow Metavanadate colorless Orthovanadate colorless Chromium(II) Cr2+ bright blue Chromium(III) Cr3+ blue-green-grey Chromium(III) hydroxide Cr(OH)63− yellowish Monochromate yellow Dichromate orange Manganese(II) Mn2+ pale pinkManganese(III) Mn3+ crimson Manganate(V) deep blue Manganate(VI) dark greenManganate(VII) (permanganate) deep purple Iron(II) Fe2+ greenish Cobalt(II) fluoride Co2+ pink Cobalt(III) amine yellow/orange Nickel(II) Ni2+ light green Nickel(II) amine complex lavender/blue Copper(I) amine complex colorless Copper(II) Cu2+ blue Copper(II) amine complex indigo-blue Copper(II) chloride blue-green Zinc(II) Zn2+ colorless Silver(I) Ag+ colorless Silver(III) in conc. HNO3 Ag3+ dark brown However, 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 Iron(III) chloride hexahydrateFeCl3·6H2O yellow/brown Iron(III) chloride anhydrate FeCl3 black Chromium (III) sulfate Cr2(SO4)3 dark green Copper(II) sulfate anhydrate CuSO4 white Copper(II) sulfate pentahydrate CuSO4·5H2O blue Copper(II) benzoate Cu(C7H5O2)2 blue Cobalt(II) chloride CoCl2 dep 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 Ammonium dichromate (NH4)2Cr2O7 orange
Ions in flame
+Colors of metal ions in flameFlame Tests at chemguide.co.uk Name Formula Color Lithium Li red Sodium Na yellow/orange Magnesium Mg brilliant white Potassium K lilac/violet Calcium Ca brick red Rubidium Rb red-violet 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 pale blue Ozone O3 pale blue Fluorine F2 pale yellow 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 testMetal Oxidizing flameReducing flameAluminiumcolorless (hot and cold), opaquecolorless, opaqueAntimonycolorless, yellow or brown (hot)gray and opaqueBariumcolorlessBismuthcolorless, yellow or brownish (hot)gray and opaqueCadmiumcolorlessgray and opaqueCalciumcolorlessCeriumred (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 opaqueMagnesiumcolorlessManganeseviolet (hot and cold)colorless (hot and cold)Molybdenumcolorlessyellow or brown (hot)Nickelbrown, red (cold)gray and opaque (cold)Siliconcolorless (hot and cold), opaquecolorless, opaqueSilvercolorlessgray and opaqueStrontiumcolorlessTincolorless (hot and cold), opaquecolorless, opaqueTitaniumcolorlessyellow (hot), violet (cold)TungstencolorlessbrownUraniumyellow or brownish (hot)greenVanadiumcolorlessgreen
References