flame test

flame test

flame test, test used in the identification of certain metals. It is based on the observation that light emitted by any element gives a unique spectrum when passed through a spectroscope. When a salt of the metal is introduced into a Bunsen burner flame, the metallic ion produces characteristic color in the flame. Some metals and the colors they produce are: barium, yellow-green; calcium, red-orange; copper salts (except halides), emerald green; copper halides or other copper salts moistened with hydrochloric acid, blue-green; lithium, crimson; potassium, violet; sodium, yellow; and strontium, scarlet. The value of this simple flame test is limited by interferences (e.g., the barium flame masks calcium, lithium, or strontium) and by ambiguities (e.g., rubidium and cesium produce the same color as potassium). A colored glass is sometimes used to filter out light from one metal; for instance, blue cobalt glass filters out the yellow of sodium.

A flame test is a procedure used in chemistry to detect the presence of certain metal ions, based on each element's characteristic emission spectrum. The color of flames in general also depends on temperature; see flame color.

The test involves introducing a sample of the element or compound to a hot, non-luminous flame, and observing the color that results. Samples are usually held on a platinum wire cleaned repeatedly with hydrochloric acid to remove traces of previous analytes. Different flames should be tried to avoid wrong data due to "contaminated" flames, or occasionally to verify the accuracy of the color. In high-school chemistry courses, wooden splints are sometimes used, mostly because solutions can be dried onto them, and they are inexpensive. Nichrome wire is also sometimes used. 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.

Sodium is a common component or contaminant in many compounds and its spectrum tends to dominate over others. The test flame is often viewed through cobalt blue glass to filter out the yellow of sodium and allow for easier viewing of other metal ions.

The flame test is fast and easy to perform, and does not require any equipment not usually found in a chemistry laboratory. However, the range of detected elements is small, and the test relies on the subjective experience of the experimenter rather than any objective measurements. The test has difficulty detecting small concentrations of some elements, while too strong a result may be produced for certain others, which tends to drown out weaker signals.

Although the test only gives qualitative information, not quantitative data about the actual proportion of elements in the sample; quantitative data can be obtained by the related techniques of flame photometry or flame emission spectroscopy. also when most of the chemicals are put in rubbing oil it makes a darker light.

Common metals

Some common metals and corresponding colors are:

Symbol Name Color
As Arsenic Blue
B Boron Bright green
Ba Barium Pale/Apple green
Ca Calcium Brick red
Cs Caesium Blue
Cu(I) Copper(I) Blue
Cu(II) Copper(II) (non-halide) Green
Cu(II) Copper(II) (halide) Blue-green
Fe Iron Gold
In Indium Blue
K Potassium Lilac
Li Lithium Red
Mg Magnesium Brilliant white
Mn(II) Manganese(II) Yellowish green
Mo Molybdenum Yellowish green
Na Sodium Intense yellow
P Phosphorus Pale bluish green
Pb Lead Blue
Rb Rubidium Red-violet
Sb Antimony Pale green
Se Selenium Azure blue
Sr Strontium Crimson
Te Tellurium Pale green
Tl Thallium Pure green
Zn Zinc Bluish green

See also


External links

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