When you hold a small amount of sodium chloride in a flame, you observe that the flame is bright yellow. what can you do to be sure that the color is due to sodium and not chlorine?

To be sure that the color is due to sodium, do a flame test. This is used to identify the presence of small number of metal ions in a compound. For sodium, the brilliant color of bright yellow is a result of an excited electron in a very high level of energy and fall to a lower level called the ground state. 

In addition, metals have different jump sizes in the energy level and this means that each different metal will have a different pattern of spectral lines, which results to different flame color.

i think it's 35 meters