When an electron in the valence shell of a metal is excited from its most stable energy level to a higher level, it tends to return back to the original ground level, by emitting certain amount of energy. During such emission, if the wavelength of the emitted radiation falls in the visible range, one can see flame colouration. This is the principle of flame colouration when certain metal salts are heated strongly in a Bunsen flame. In order to have a sufficiently visible flame, the salt must spread well into the flame. This is achieved through volatilization of the salt in flame temperature. The greater the volatility of the salt, the better flame colouration created. Of all the salts of metals, chloride salts are the most volatile ones. That is the reason all metal salts (including carbonates) are first converted to their chlorides by dipping them into concentrated HCl, prior to doing flame test. This procedure of immersing in concentrated HCl has nothing to do with neutralization.