As this year comes to an end and those of us in the Northern Hemisphere approach the darkest time of our endless journey around our humble central star, what better way to light up your classroom than with the brilliant colors of the flame test lab. This activity combines everything that students love: lab work, flames and color. It’s a must for your high school science class.
Just like every person has a unique finger print, the color of light emitted by a metal when heated in a flame is unique to that metal. The flame’s test colors can be seen immediately, they are easy to identify and brilliant in their effect. Every year my students thank me for the chance to do this lab…at least until they remember that there is a lab write-up that they must complete.
In this lab students will try to identity the following six metal salts based on their emission colors:
|Name of Metal Salt||Chemical Formula||Flame Test Color||Representative Wavelength|
|Copper chloride||CuCl2||Blue-Green||490 nm|
|Potassium chloride||KCl||Light Violet||410 nm|
|Lithium chloride||LiCl||Red-Orange||625 nm|
|Strontium chloride||SrCl2||Red||650 nm|
|Sodium chloride||NaCl||Yellow||580 nm|
|Calcium chloride||CaCl2||Orange||600 nm|
This lab is easy to set up and there are no chemicals to mix. The minimal equipment requirements include the metal salts, Bunsen burners, cotton swabs (Q-tips), a few beakers and of course safety goggles. If you don’t have the salts, consider buying the affordable kit from Flinn Scientific.
When the metals are placed in the flame the electrons in each atom absorb energy. This energy is used to move the electrons from their ground state to an excited state. When the electrons move back to a lower energy level they emit the absorbed energy as a particle of light called a photon (see diagram at right).
Because every element has a different electron configuration, the energy of the emitted photons will be different and therefore a different color will be emitted by each metal. Using the representative wavelength in the table above and the following equations, students can convert the wavelengths to energies in joules.
c = f ⋅ λ
E = h ⋅ f
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