# 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, $\lambda$ 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

Name Formula Color
Alkali metals M+ Colorless
Alkaline earth metals M2+ Colorless
Scandium(III) Sc3+ Colorless
Titanium(III) Ti3+ Violet
Titanyl TiO2+ Colorless
Chromium(III) Cr3+ Blue-green
Chromate CrO42- Colorless or Yellow(sometimes)
Dichromate Cr2O72- Orange
Manganese(II) Mn2+ Light Pink
Manganate(VII) (Permanganate) MnO4- Deep violet
Manganate(VI) MnO42- Dark green
Manganate(V) MnO43- Deep blue
Iron(II) Fe2+ Pale green
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+ Colorless

[2] 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
Copper(II) benzoate C14H10CuO4 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

### 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
Chlorine Cl2 greenish yellow
Iodine I2 purple
Nitrogen Dioxide NO2 brown
Dinitrogen tetroxide N2O4 colorless

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.

Metal[3] Oxidising 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
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), violet (cold)
Tungsten colorless brown
Uranium Yellow or brownish (hot) green