Why are chlorides used in flame test?

There are two reasons why chlorides are used in flame tests.
1. Metal chlorides are usually more volatile than other salts.
More of the salt will be converted to vapour in the flame and therefore give a brighter colour.
2. Chlorine atoms do not emit energy in the visible region.
In the flame test, electrons in both the metal atoms and the chlorine atoms are excited to higher energy levels.
These electrons emit energy of characteristic wavelengths when they return to the ground state.
For most metals, the emitted light is in the visible region.
For chlorine atoms, the emitted energy is not in the visible region.
Thus, the flame colour is caused by the metal alone.