Introduction
The flame test is one of the most commonly used analytical processes in chemistry. It is widely used to detect and analyze the presence of certain elements in the given salt or compound. Primarily, the flame test detects the presence of metal ions in a compound, and as ions of each element have a specific characteristic based in their emission spectrum, the flame test for every element is different and distinctive.
This distinction is shown by the color of flames given out when the salt containing certain metal ions is burnt.
It is important to note here that the emission spectrum of each element that determines the flame color involves atoms instead of ions. The transition of electrons in the atoms tends to produce the visible color lines which are seen in the flame test.
Chemistry Behind Flame Test
The chemistry behind the flame test is simple. As we know that when an atom or ion is excited by heating to high temperatures, the electrons are promoted from their normal unexcited state into other orbitals, known as higher orbitals, as they have higher energy as compared to the normal or ground state orbitals.
When these excited electron falls back down to lower levels which can happen simultaneously or in several steps, the energy they have absorbed is released. This energy is released in the form of light.
Each jump involves the release of a specific amount of energy in the form of light. And each transition from higher to lower orbital corresponds to a frequency or wavelength.
All these jumps or transitions result in the formation of a spectrum of lines. Some of these lines are part of the visible part of the spectrum.
And the final color we see is a combination of all these individual colors. This color is the distinctive color of the element we observe in the flame test.
For instance, in the case of potassium or sodium ions or many other metal ions, the transition of electrons involves very high energies. This result in lines that fall in the UV part of the spectrum which is invisible to the naked eye.
This explains the role of atoms rather than ions to be the basis of flame test.
And the jumps we can see in flame tests are due to falling of electrons from a higher to a lower level in the metal atoms.
When we put sodium chloride, containing sodium ions, into a flame, the sodium atoms are formed as a result of certain sodium ions that regain their electrons and produce neutral sodium atoms again.
The orbitals and their configuration are very important features in each element with respect to a flame test.
The structure of the unexcited state of sodium atom 1s22s22p63s1 and within the flame, there are different sorts of excited states of the electrons.
Sodium gives a bright orange-yellow flame color. This results from promoted electrons falling back from the 3p1 level to their normal 3s1 level.
The exact size of the potential energy jumps varies from one metal to another.
This means that each metal will have a different pattern of spectral lines, and so have a distinct flame color.
The elements of the Group1 are the easiest metals that can be accurately identified using the flame test.
For other metals, flame test does not provide a definitive identification, however, it gives a general idea of the probable compound.
Practical Process Detail
The procedure of this test is simple as it involves introducing sample of the compound or element to a non-luminous, hot flame, and examining the color of the resulting flame.
The flame test is an easy experiment to set up and is often conducted in science classes.
The principle of the test is that the atoms of the sample evaporate and as they are hot, they give off light when present in the flame.
A mixture of samples of a large amount of sample can also emit light. But such light is not good for identification analysis.
As described earlier, the individual atoms of a sample that are present in the flame emit light due to the transitions of electron between different atomic energy levels. Such transitions emit light that has very specific frequencies, and which is the characteristic of the chemical element.
Hence, the flame gets the color. And it is determined by the characteristics and properties of the chemical element of the material that is introduced into the flame.
There are certain points that need to be followed to obtain precise results in a flame test.
For instance, the samples are carried on a platinum wire, which is repeatedly cleaned with hydrochloric acid (HCl) to remove traces of any elements.
The compound to be assessed is usually made into a paste with concentrated hydrochloric acid, as it is volatile, and give good quality results.
It is also recommended to use different flames to avoid errors in the results due to contaminated flames, and to confirm the precision of the color.
The presence of sodium is considered as a common component in many compounds. And its spectrum is likely to dominate the light spectrum of other elements. To avoid this, the test flame is often viewed using a cobalt blue glass that filters out the yellow of sodium and allows the accurate presentation of color of other metal.
Flame Test Results of Common Elements
Here is the list of most common elements that are detected through the flame test. They have a distinct emission spectrum that allows them to show specific colored flame in a flame test. However, the colors given in the table are only a guide as colors are perceived differently by different people.
| Sr # | Element | Symbol | Color |
| 1 | Sodium | Na | Intense Yellow |
| 2 | Potassium | K | Intense Yellow Orange |
| 3 | Calcium | Ca | Orange Red |
| 4 | Lithium | Li | Red |
| 5 | Strontium | Sr | Red |
| 6 | Barium | Ba | Pale green |
| 7 | Copper | Cu | Blue-green |
| 8 | Cesium | Cs | Blue |
| 9 | Iron | Fe (II) | Gold |
| 10 | Germanium | Ge | Pale blue |
| 11 | Mercury | Hg | Red |
| 12 | Nickel | Ni | Silver-white |
| 13 | Hafnium | Hf | White |
| 14 | Phosphorus | P | Pale bluish green |
| 15 | Molybdenum | Mo | Yellowish Green |
| 16 | Manganese | Mn | White |
| 17 | Tin | Sn | Blue-white |
| 18 | Zinc | Zn | Colorless or bluish-green |
| 19 | Lead | Pb | Bluish white |
| 20 | Zirconium | Zr | Light red |
| 21 | Radium | Ra | Crimson |
| 22 | Chromium | Ch | Silver-white |
| 23 | Cadmium | Cd | Brick red |
| 24 | Arsenic | As | Blue |
Certain precious metals, including platinum, titanium, palladium, gold, and silver do not produce a distinctive flame color. However, some can produce sparks when exposed to hot flame.
Safety Notes
The flame test can be dangerous if proper protocol and safety measures are not taken. It is advised to use good safety techniques. We should wear a chemical apron and good quality chemical splash resistant goggles. It is also important that we practice the flame test under the supervision of a teacher.
References
https://www.soinc.org/sites/default/files/uploaded_files/flametest.pdf