Q.In flame test, Barium gives apple green colour. Give reason.

The flame's heat excites barium's outer electrons; as they fall back to lower energy states they emit visible light, and for barium this emitted light corresponds to the apple-green region of the spectrum.

When a volatile barium salt is introduced into a Bunsen flame, the thermal energy of the flame excites the loosely-held outer (valence) electrons of the Ba2+ ion from their ground state to higher, unstable energy levels. Since this excited state is unstable, the electrons quickly fall back to their original, lower energy levels (or to the ground state), releasing the absorbed energy in the form of a photon of visible light. The exact colour observed depends on the specific spacing between the energy levels available to that particular element, which is unique to each element's electroni …