Question

what energy source causes excitation of electrons with the metal salts?

Answer 1

In the metal salt burning experiment the thermal enengy raises the electrons to a higher energy state. The thermal energy produces collisions between the atoms causing the atom's electrons to be excited. So, the thermal energy is the source that causes excitation of electrons within the metal salts. Light is emitted when the excited electrons return to the ground state, it is called electron relaxation. Each metal salt has a characteristic light colour that it releases.