Definitions

# Colors of chemicals

All chemical compounds have colors, and in some cases these are distinctive or useful for identification. The study of chemical structure by means of energy adsorption and release is generally referred to as spectroscopy.

## Theory

All chemical compounds, including atoms, are capable of absorbing and releasing energy. The amount(s) of energy (quanta) absorbed and released is determined by the quantum structure of the chemical. The release of energy visible to the human eye spans the wavelengths 380 nm to 760 nm and is commonly referred to as color. The relationship between energy and wavelength is determined by the equation:
$E = hf = frac\left\{hc\right\}\left\{lambda\right\} ,!$
where E is the energy of the quanta (photon), h is Planck's constant, $lambda$ is the wavelength and c is the speed of light. The relationship between chemical structure and energy can be understood using atomic orbital, molecular orbital, or Ligand Field Theory. In organic compounds the color can be determined by the difference between the Highest Occupied Molecular Orbital and the Lowest Unoccupied Molecular Orbital. The energy absorbed and/or released does not directly correlate to what humans perceive as color. The absorption of a particular wavelength of light effectively subtracts it from the full visible spectrum, and what we see is the complementary color, made up of the other visible wavelengths. 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.

Wavelength (nm) Color Complementary Color
400-424 Violet Green-yellow
424-491 Blue Yellow
491-570 Green Red
570-585 Yellow Blue
585-647 Orange Green-Blue
647-700 Red Green

## Examples

### Ions in aqueous solution

Name Formula Color
Alkali metals M+ None
Alkaline earth metals M2+ None
Scandium (III) Sc3+ None
Titanium (III) Ti3+ Violet
Titanyl TiO2+ None
Chromate CrO4 2- 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+ Light blue
Iron (III) Fe3+ Yellow/brown
Cobalt (II) Co2+ Light red
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
Zinc (II) Zn2+ None
Silver Ag+ None

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 redish 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 Blue
Copper (II) sulfate pentahydrate CuSO4 · 5H2O 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

### Oxidising Metals

Flame Tests on cations for Alkali and Alkali Earth Metals

Name Formula Color
Potassium K Lilac/Purple
Sodium Na Yellow
Lithium Li Red
Cesium Cs Blue
Calcium Ca Red/Orange
Strontium Sr Red
Barium Ba Green/Yellow

### Oxidising Gases

Name Formula Color
Hydrogen H2 Colorless

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