The flame test is an analytical chemistry technique that helps identify elements in samples based on their characteristic emission spectra. Mostly the flame test detects metal ions, but some nonmetals color flames as well.
How the Flame Test Works
The basic premise is that heat from a flame gives atoms enough energy that their electrons become excited. Dropping to a more stable energy state involves the release of photons. These photons have a frequency (light color) that is a characteristic of the element.
However, not all elements release light in the visible portion of the spectrum. Some elements don’t change a flame’s color at all. For example, gold, silver, platinum, and palladium do not yield a flame test result. However, some of these metals produce sparks in a flame and other deposit pure metal onto a surface.
Advantages and Disadvantages of the Flame Test
The flame test offers both advantages and disadvantages as an analytical technique.
Advantages
- Extremely quick and easy
- Only requires a tiny sample
- Good at eliminating possible elements in a sample
- Visually appealing, so it’s great at raising student interest in science
Disadvantages
- Does not definitively identify a sample
- Results are subjective
- Results are highly susceptible to contamination, particularly from sodium
- Several elements yield approximately the same color results
- Some samples yield brighter colors than others
- Results vary somewhat depending on the exact chemical composition of the sample and the fuel
- Qualitative rather than quantitative technique
- Does not work with extremely dilute samples
The bead test is a related technique. Better techniques include flame photometry, flame emission spectroscopy, and flame absorption spectroscopy. However, these methods are quite a bit more expensive.
How to Do the Flame Test
There are several ways of performing the flame test.
- Dissolve the sample in water or another solvent, soak a wooden splint in the liquid, and let it dry.
- Dip a Nichrome wire into a solid or liquid sample.
- Make a paste of a solid sample with hydrochloric acid (HCl) and dip a splint or wire into the paste.
- Dip a cotton swab into the sample. (This method is prone to sodium contamination.)
- Dissolve the sample in a small amount of methanol. Dip a bit of melamine sponge (e.g., Magic Eraser) into the sample.
Options for the flame include a candle flame, Bunsen burner flame, or gas flame.
Basically, you dip a wire or splint into a solid sample or its solution and expose the sample to a colorless flame. Viewing the results through a cobalt blue glass filters out excess yellow and makes identification a bit easier. Once you have a color, compare it to a table of flame test colors.
Make Colored Fire
The flame test is the basis for firework colors, colored fire spray bottles, and colored campfires.
Table of Flame Test Colors
This is a table of flame test colors, ordering the elements alphabetically by symbol.
| Symbol | Name | Color |
|---|---|---|
| Al | Aluminum | Silvery-white |
| As | Arsenic | Blue |
| B | Boron | Bright green |
| Ba | Barium | Light apple green |
| Be | Beryllium | White |
| Bi | Bismuth | Azure blue |
| C | Carbon | Orange |
| Ca | Calcium | Brick red |
| Cd | Cadmium | Brick red |
| Ce | Cerium | Yellow |
| Co | Cobalt | Silvery white |
| Cr | Chromium | Silvery white |
| Cs | Cesium | Blue violet |
| Cu(I) | Copper(I) | Blue-green |
| Cu(II) | Copper(II) | Green (non-halide) to blue-green (halide) |
| Ge | Germanium | Pale blue |
| Fe(II) | Iron(II) | Gold |
| Fe(III) | Iron(III) | Orange brown |
| H | Hydrogen | Pale blue |
| Hf | Hafnium | White |
| Hg | Mercury | Red |
| In | Indium | Indigo blue |
| K | Potassium | Lilac |
| Li | Lithium | Carmine red |
| Mg | Magnesium | Colorless to white |
| Mn(II) | Manganese(II) | Yellowish green |
| Mo | Molybdenum | Yellowish green |
| Na | Sodium | Bright yellow |
| Nb | Niobium | Green or blue |
| Ni | Nickel | Colorless to silvery white |
| P | Phosphorus | Pale blue-green |
| Pb | Lead | Blue-white |
| Ra | Radium | Crimson red |
| Rb | Rubidium | Violet red |
| Sb | Antimony | Pale green |
| Sc | Scandium | Orange |
| Se | Selenium | Azure blue |
| Sn | Tin | Blue-white |
| Sr | Strontium | Crimson or scarlet red |
| Ta | Tantalum | Blue |
| Te | Tellurium | Pale green |
| Ti | Titanium | Silver-white |
| Tl | Thallium | True green |
| V | Vanadium | Yellowish green |
| W | Tungsten | Green |
| Y | Yttrium | Red: carmine, scarlet, or crimson |
| Zn | Zinc | Colorless to blue-green |
| Zr | Zirconium | Dull red |
References
- Barrow, R. F.; Caldin, E. F. (1949). “Some Spectroscopic Observations on Pyrotechnic Flames”. Proceedings of the Physical Society. Section B. 62 (1): 32–39. doi:10.1088/0370-1301/62/1/305
- Landis, Arthur M.; Davies, Malonne I.; Landis, Linda; Thomas, Nicholas C. (2009). “‘Magic Eraser’ Flame Tests”. Journal of Chemical Education. 86 (5): 577. doi:10.1021/ed086p577
- Patnaik, Pradyot (2002). Handbook of Inorganic Chemicals. McGraw-Hill. ISBN 0-07-049439-8.
- Sanger, Michael J.; Phelps, Amy J.; Banks, Catherine (2004). “Simple Flame Test Techniques Using Cotton Swabs”. Journal of Chemical Education. 81 (7): 969. doi:10.1021/ed081p969