Aflame test is relatively quick test for the presence of some elements in a sample. The technique is archaic and of questionable reliability, but once was a component ofqualitative inorganic analysis. The phenomenon is related topyrotechnics andatomic emission spectroscopy.[1] The color of theflames is understood through the principles ofatomic electron transition andphotoemission, where varying elements require distinct energy levels (photons) for electron transitions.[2][3]
Robert Bunsen invented the now-famousBunsen burner in 1855, which was useful in flame tests due to its non-luminous flame that did not disrupt the colors emitted by the test materials.[4][1] TheBunsen burner, combined with a prism (filtering the color interference ofcontaminants), led to the creation of thespectroscope, capable of emitting thespectral emission of various elements.[1] In 1860, the unexpected appearance ofsky-blue anddark red was observed inspectral emissions byRobert Bunsen andGustav Kirchhoff, leading to the discovery of twoalkali metals,caesium (sky-blue) andrubidium (dark red).[4][1] Today, this low-cost method is used in secondary education to teach students to detectmetals in samples qualitatively.[2]
A flame test involves introducing a sample of the element or compound to a hot, non-luminous flame and observing the color of the flame that results.[4] The compound can be made into a paste with concentrated hydrochloric acid, asmetal halides, being volatile, give better results.[5] Different flames can be tried to verify the accuracy of the color. Wooden splints,Nichrome wires,platinum wires,magnesia rods, cotton swabs, andmelamine foam are suggested for support.[6][7][8] Safety precautions are crucial due to theflammability andtoxicity of some substances involved.[9][10][11][6] When using a splint, one must be careful to wave the splint through the flame rather than holding it in the flame for extended periods, to avoid setting the splint itself on fire. The use of acotton swab ormelamine foam (used in “eraser” cleaning sponges) as a support has also been suggested.[7][8][6]Sodium is a common component orcontaminant in many samples,[2] and its spectrum tends to dominate many flame tests others.[5] The test flame is often viewed throughcobalt blue glass to filter out the yellow of sodium and allow for easier viewing of other metal ions.[citation needed]
The color of the flames also generally depends ontemperature andoxygen fed; seeflame colors.[5] The procedure uses differentsolvents andflames to view the test flame through acobalt blue glass ordidymium glass to filter theinterfering light ofcontaminants such assodium.[12]
Flame tests are subject of a number of limitations. The range of elements positively detectable under standard conditions is small. Some elements emit weakly and others (Na) very strongly.Gold,silver,platinum,palladium, and a number of other elements do not produce a characteristic flame color, although some may produce sparks (as do metallictitanium andiron); salts of beryllium and gold reportedly deposit pure metal on cooling.[12] The test is highlysubjective.
In flame tests, ions areexcited thermally. These excited states then relax to the ground state with emission of a photon. The energy of the excited state(s) and associated emitted photon is characteristic of the element. The nature of the excited and ground states depends only on the element. Ordinarily, there are no bonds to be broken, and molecular orbital theory is not applicable. The emission spectrum observed in flame test is also the basis offlame emission spectroscopy,atomic emission spectroscopy, andflame photometry.[4][13]
Some common elements and their corresponding colors are:
| Symbol | Name | Color[5] | Image |
|---|---|---|---|
| Al | Aluminium | Silver-white, in very high temperatures such as an electric arc, light blue | |
| As | Arsenic | Blue |  |
| B | Boron | Bright green |  |
| Ba | Barium | Light apple green |  |
| Be | Beryllium | White | |
| Bi | Bismuth | Azure blue | |
| Ca | Calcium | Brick/orange red; light green as seen through blue glass. |  |
| Cd | Cadmium | Brick red | |
| Ce | Cerium | Yellow | |
| Co | Cobalt | Silvery white |  |
| Cr | Chromium | Silvery white |  |
| Cs | Caesium | Blue-violet |  |
| Cu(I) | Copper(I) | Blue-green | |
| Cu(II) | Copper(II) (non-halide) | Green |  |
| Cu(II) | Copper(II) (halide) | Blue-green | |
| Fe(II) | Iron(II) | Gold, when very hot such as an electric arc, brightblue, or green turning to orange-brown | |
| Fe(III) | Iron(III) | Orange-brown |  |
| Ge | Germanium | Pale blue | |
| H | Hydrogen | Pale blue | |
| Hf | Hafnium | White | |
| Hg | Mercury | Red | |
| In | Indium | Indigo blue | .jpg) |
| K | Potassium | Lilac (pink); invisible throughcobalt blue glass (purple) |  |
| Li | Lithium | Carmine red; invisible throughgreen glass |  |
| Mg | Magnesium | Colorless due to Magnesium Oxide layer, but burning Mg metal gives an intensewhite | |
| Mn(II) | Manganese(II) | Yellowish green |  |
| Mo | Molybdenum | Yellowish green | |
| Na | Sodium | Bright yellow; invisible throughcobalt blue glass. See alsoSodium-vapor lamp |  |
| Nb | Niobium | Green or blue | |
| Ni | Nickel | Colorless to silver-white |  |
| P | Phosphorus | Pale blue-green | |
| Pb | Lead | Blue-white |  |
| Ra | Radium | Crimson red | |
| Rb | Rubidium | Violet red |  |
| S | Sulfur | Blue |  |
| Sb | Antimony | Pale green |  |
| Sc | Scandium | Orange | |
| Se | Selenium | Azure blue | |
| Sn | Tin | Blue-white | |
| Sr | Strontium | Crimson to scarlet red; yellowish through green glass and violet through blue cobalt glass |  |
| Ta | Tantalum | Blue | |
| Te | Tellurium | Pale green | |
| Ti | Titanium | Silver-white | |
| Tl | Thallium | Pure green | |
| V | Vanadium | Yellowish green | |
| W | Tungsten | Green | |
| Y | Yttrium | Carmine, crimson, or scarlet red | |
| Zn | Zinc | Colorless to blue-green |  |
| Zr | Zirconium | Mild/dull red |
Media related toFlame test at Wikimedia Commons