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Flame test

 

Under normal conditions the atoms are in the ground state, which is the most stable thermodynamically. However, if we heat them absorb energy and thus reach an excited state. This State has a certain energy, which is characteristic of each substance. The atoms in an excited state have a tendency to return to the ground state, which is energetically more favorable. To do this they must lose energy, for example, in the form of light. Possible excited States are unique for each element, and the ground state is always the same, the radiation emitted is also peculiar for each item and can therefore be used to identify you. This radiation depends on the difference between the excited States and the key according to Planck's law:

AE = hv;

AE = difference of energy among excited and key States
h = Planck's constant (6.62 10-34 J s). v = frequency

Therefore, the emission spectrum can be considered as a "fingerprint" of an element. This fact was known already since antiquity, before even as was the case, so chemists have used the "trials to the flame" as a simple method of identification. Currently, there are analysis techniques based on this principle, such as Atomic emission spectroscopy, allowing us to not only identify, but quantify the presence of different elements.

 

 

 

 

 

Then indicate the colors of the trials to the flame of some elements:

Calcium: red flame, copper: green flame, sodium: orange flame, lithium: pink flame, potassium: violet flame, barium: pale green flame and lead: blue flame.

Copper flames
   

Reproducir
flame test

 

Lithium flames
   

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The videos show the flames produced when burning ethanol adding a quantity of1) copper sulphate  and 2) lithium nitrate.


           

Department of Inorganic Chemistry


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