Flame Colouration

Flame colouration is a method used for mineral identification. It is based on the principle that the movement of electrons causes the colour in metal and releases energy in the form of light. The colours of this light change based on the metals used. Larger atoms emit colours with lower energy, whereas smaller atoms produce colours with greater energy. 

The spectral lines of various metals have distinct patterns, and it is also one of the reasons for the formation of different flame colours. 

Flame colouration importance

Scientists use flame colouration to identify components in unknown compounds. This information helps them identify the molecule and determine if it contains metalloids or metal ions.

Origin of flame colours

It is possible to move electrons from lower orbitals of an atom or ion to higher ones by heating them to a high temperature. As they fall to lower levels (either all at once or over a period of time), they release energy in the form of light. Each of these jumps releases a certain quantity of energy as light energy at a given wavelength (or frequency). These jumps form a spectrum of lines, some of which are visible. It comprises a mix of colours.

The high energies of Sodium (or other metal) ions cause lines in the UV spectrum that your eyes cannot see. Electrons fall from a higher to lower level in the metal atom, which causes the jumps you see in flame tests. When you put Sodium chloride in a flame, certain Sodium ions gain electrons and become neutral Sodium atoms when heated. Although the structure of a Sodium atom in its unexcited state is 1s22s22p63s1, there will be several excited states of electrons within the flame. Promoted electrons fall back down from the 3p1 level to their normal 3s level, providing Sodium’s flame a bright orange-yellow colour.

The size of the energy jumps varies from metal to metal, which implies that each metal has a unique spectral line pattern and hence a distinct flame colour. Electrons’ movements in metal ions generate flame colours, and they have the electron configuration 1s22s22p6. A flame can heat an electron, giving it more energy. Depending on how much energy the electron gets from flame, it can move it to a higher orbital, like 7s or 6p or 4d. The electron’s higher and more unstable state causes it to fall back to its original state, but not necessarily in one transition.

What are some common flame colouration examples?

Sodium produces a yellow flame, lithium and strontium a red flame, calcium an orange flame, copper a blue flame, and barium a green flame are examples of flame colouration.

What is the Flame Test?

The flame test is a common analytical method in Chemistry, and it detects and identifies components in a chemical or salt. The flame test detects metal ions in a compound, and each element’s ions have a different feature based on their emission spectrum, so the flame test is different.

The colour of the flames produced when the burnt metal ion salt shows this distinction. Each element’s emission spectrum contains atoms rather than ions. The colour lines seen in flame tests are due to electrons converting into ions.

Chemistry Behind the Flame Test

The flame test chemistry is simple. When we heat an atom or ion to a high temperature, the electrons promote from their regular unexcited state into higher orbitals, containing more energy than the normal or ground state orbitals.

The energy released when these excited electrons return to lower levels might happen simultaneously or in steps, and this energy emits as light.

Each leap releases a set quantity of light energy and each orbital transition corresponds to a frequency or wavelength.

These transitions or jumps result in a spectrum of lines. These lines appear in the spectrum.

The final colour we see is a mix of all these colours, and it is the distinctive colour of the element that we see in the flame test.

For example, electron transitions occur at extremely high energies in potassium, Sodium, and many other metal ions, resulting in lines invisible to the human eye in the UV spectrum. It explains why atoms rather than ions are used in the flame test.

Electrons in metal atoms fall from higher to lower levels, causing jumps in flame tests. When Sodium chloride, which contains Sodium ions, is heated to high temperatures, some ions regain their electrons and form neutral Sodium atoms. Each element’s orbitals and configuration are critical features in a flame test.

The unexcited Sodium atom 1s22s22p63s1 and the excited electron states within the flame. Sodium produces an orange-yellow flame, and it occurs when promoted electrons back to their normal 3s1 level.

The size of the potential energy jumps varies from metal to metal. Each metal has a unique pattern of spectral lines and hence a unique flame colour.

Group 1 metals are the easiest to identify using the flame test. The flame test gives an idea of the probable compound and not a definitive identification for other metals.

Limitations of the flame test

• As long as the ions’ concentration is low, the flame test will not detect them.
• The light intensity varies among samples. For example, yellow Sodium emissions are substantially more intense than red litmus emissions during the flame test.
• The presence of contaminants or impurities in the sample affects the flame test. For example, Sodium is present in most compounds and provides the flame yellow colour. 
• The flame test cannot distinguish all elements. Most metals create similar colours, whereas other compounds cause no colour change.

Conclusion

One of the most used analytical techniques in chemistry is the flame colouration test. It is a popular method for detecting and analysing the presence of certain components in salts or compounds. The flame test determines the presence of metal ions in a compound, and since each element’s ions have distinctive features based on their emission spectrum, the flame test for each element is different. The colour of flames produced when burning salt-containing particular metal ions shows this distinction. It is worth noting that the emission spectrum of each element determines the flame colour and not the ions. The visible colour lines seen in the flame test are due to the transition of electrons in the atoms.