- 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.
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:
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
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.
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. Charge-transfer complexes tend to have very intense colors for different reasons.
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 Vanadium(II) V2+ Lavender Vanadium(III) V3+ Dark grey/green Vanadyl VO2+ Blue Pervanadyl VO2+ Yellow Metavanadate VO3- None Orthovanadate VO43- None Chromium(III) Cr3+ Blue-green Chromate CrO4 2- Colorless or Yellow(sometimes) Dichromate Cr2O72- Orange Manganese(II) Mn2+ Colourless 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+ 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+ 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 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'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 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
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
Name Formula Color Hydrogen H2 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 fusses and the color of the resulting bead observed.
Metal Oxidising flame Reducing flame Aluminum 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) 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), biolet (cold) Tungsten colorless brown Uranium Yellow or brownish (hot) green Vanadium colorless green
- ^ http://www.sapdesignguild.org/resources/glossary_color/index1.html
- ^ CRC Handbook of Chemistry and Physics. CRC Press. 1985. ISBN 0849304660.
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