Nov. 29, 2018

The Colours of Firework

Experiment of the Month

  • Video of the experiment (Credit: Maximilian Ruppert)
  • CuCl Credit: Maximilian Ruppert
  • CuS04 Credit: Maximilian Ruppert
  • NaCl Credit: Maximilian Ruppert
  • LiCl Credit: Maximilian Ruppert
  • Energy Levels

When we think of New Year’s Eve, the first thing that comes to mind, of course, are the fireworks. Loud popping and colorful lights in the night sky. What happens exactly? We want to explain this to you today. The colorful lights are usually alkali metal salts, but also other compounds that are heated by the explosion of propellant (often black powder) and dispersed in the night sky. Depending on the alkali metal used, other colors occur. This is called flame coloring.


Magnesia grooves, Teclu or Bunsen burner, water, lithium chloride, sodium chloride, copper (I) chloride and copper (II) sulfate.


The magnesia grooves are moistened with water and then dipped in about 0.5g of one of the salts, so that the salt adheres to the magnesia. The burner is ignited and set to a roaring flame.
Then the Magnesia groove is held in the cone of the burner. The experiment is over when the flame is no longer colored.


It is essential to wear protective goggles during the experiment. If possible, the experiment should be carried out in the fume cupboard. Already used Magnesia grooves must be purified over the flame of a burner for 5 minutes.


Since multiple chloride ions were used and yet different flame colors were observed, it can be concluded that the anions do not contribute, or only to a small extent, to the flame coloration. So the respective cations Li+, Na+, Cu+ and Cu2+ should be responsible. In the burner flame, metal ions are reduced to atoms. The high temperature (over 1200 °C) now lifts individual electrons of the metals to a higher energy level, which is further away from the atomic nucleus. When the electrons return to their original energy level, they release the previously absorbed energy in the form of a light quantum (= a photon). Depending on the amount of absorbed energy, light quanta of different wavelengths are emitted, therefore the elements differ in their flame color.


If one wants to describe the position of an electron, one speaks in the Bohr atomic model of energy levels.

The Bohr atomic model, however, applies only to the hydrogen atom. Because this has only one electron. For all other atoms you have to consult quantum mechanics. Here one no longer speaks of orbits on which the electrons move, but of orbitals. An orbital describes the probable location of an electron.
Depending on the electron configuration (number and distribution of electrons within an atomic nucleus) different electrons of an atom are excited. This means that they move further away from the core. When they return to their original energy level, they release the energy they have previously absorbed in the form of a light quantum (= a photon). Depending on the amount of energy, light quanta of different wavelengths are emitted, therefore the elements differ in their flame color.

See Fig. 5 for a schematic representation of energy levels.

The 3s electron of sodium is raised to the energy level 3p and then "falls" back to its original energy level. It releases the energy in the form of orange light with a wavelength of 589nm. The light of this wavelength emitted by sodium atoms is also called the D-Frauenhofer line, because Joseph von Frauenhofer discovered it in the line spectrum sun. Gustav Robert Kirchhoff and Robert Bunsen later concluded that there must be sodium atoms in the sun. In inorganic chemistry, flame coloring is used as the first experiments to gain an overview of the substances contained. However, as sodium is found practically everywhere, it often covers almost all the other colors of a sample. That's why you make do with a cobalt glass, which filters out the orange. A much more accurate form of analysis is atomic absorption spectrometry. Here, a sample is introduced into a flame and the emitted radiation is measured with a detector. Not only emissions in the visible range but also radiations can be measured, which the human eye can not recognize.



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