Why Does Flame Change Color?

Author: Leon Zhou

Editor: Kira Tian

Artist: Austin Xu

If you have ever done chemistry experiments in the laboratory, you might have noticed that when metals are heated, the flame sometimes changes color. For example, when heating reddish-brown copper, the flame will show a color of green! A phenomenon like this is called a flame reaction. To understand how a flame reaction occurs, the concept of “electron transition” should be understood.

For every atom, there consists of a positively charged nucleus and negatively charged electrons. With the current model of an atom, the nucleus is located in the middle of the atom with electrons dispersed around the nucleus in different “shells”. A higher level of shell possesses greater energy.

When electron transition occurs, the electron absorbs energy and transits from a lower energy level to a higher energy level, moving from a lower-level shell to a higher-level shell. For example, when an electron in a Hydrogen atom absorbs energy, it may “jump” from the 1s orbital to the 2p orbital. So how does electron transition relate to the phenomenon of flame reaction?

When metal is heated, the electron absorbs energy and transits to a higher energy level. But electrons tend to stay in the lowest energy level to achieve stability. When the electrons transit back to the lower energy level, it releases energy in the form of visible light! The amount of energy released can be expressed as the following equation: E=-2.179J (E= energy) Z represents the number of protons in the atom, and n represents the energy level of electrons. Using this equation, it is expected that different metals will release different amounts of energy due to variations in Z and n.

Finally, energy is released in the form of radiation from electron transition. The frequency of such radiations depends on the amount of energy released. When the frequency of the radiation falls into the spectrum of visible light, a “change in color” of flame occurs.


Flame tests, Chemistry LibreTexts


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