Flame test
Under normal conditions, atoms are in the ground state, which is the most thermodynamically stable. However, if we heat them, it absorbs energy and thus reaches an excited state. This state has a certain energy, which is characteristic of each substance. Atoms in an excited state have a tendency to return to the ground state, which is energetically more favourable. To do this, they must lose energy, for example in the form of light. Since the possible excited states are peculiar to each element and the ground state is always the same, the emitted radiation will also be peculiar to each element and therefore can be used to identify it. This radiation will depend on the difference between the excited states and the ground state, according to Planck's law:
AE = hν ;
AE = energy difference between the excited and ground states
h = Planck's constant (6.62 10-34 J s)
ν= frequency
Therefore, the emission spectrum can be considered as the “fingerprint” of an element. This fact was known since ancient times, even before understanding how it happened, so chemists have used "flame tests" as a simple method of identification. Currently, there are analysis techniques based on this principle, such as atomic emission spectroscopy, which allow us not only to identify, but also to quantify the presence of different elements. Below we indicate the colours of the flame tests 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.
The videos show the flames produced by burning ethanol to which a certain amount of 1) copper sulfate and 2) lithium nitrate has been added.
