A student recently asked an interesting question What happens when KCl is introduced in the Bunsen flame?. This apparently simply question led me to an interesting debate. We all know that emission of characteristic colors of alkali metal salts especially the halides and carbonates in ordinary Bunsen flames comes from the atomic emission not their oxy compounds (like the alkaline earth salts) or their ions.
The point of debate is point number 3:
- KCl solution has to atomize and become KCl (solid).
- We would get KCl in gas phase from the heat of the flame.
- How does KCl (gas) convert to K atoms in a flame? What is the path? Note Thermal decomposition of salts without a flame is another story. Please do not mix, decomposition in a flame versus isolated vapor phase studies of salts.
Flame chemistry is quite complex and my understanding is that flames are electrically conducting from a very small amount of free electrons and ions. Certainly, we are not talking about cool flames like candles. Flame conductivity is a well known phenomenon and it is utilized in gas chromatography's "flame ionization detector" as well.
Herein, I am talking about spectroscopic flames used in atomic emission spectroscopy. It is known for more than 100 years that when we introduce alkali metal salts into the Bunsen flames, the conductivity of flame increases as a function of salt concentration. For alkali halides, it is a square root function of concentration. This clearly shows that there are free ions and electrons in a flame in which alkali metal salts are introduced. The amount must be really small but significant enough to alter flame properties Electrical properties of flames.
There must be certainly thermal decomposition but it is not a complete story at all. The key question is the path of KCl(s) to K atoms!
(a) KCl goes to potassium ions in the flame, by thermal decomposition, and it becomes neutral potassium atom in flame after gaining an electron (from flame components)?
(b) KCl is thermally decomposed directly to potassium atoms and there are no intermediate potassium ions?