Question

Why does only one of these ions (Co, Cu, or Ni) produce a positive flame test?

Answer 1

Useful flame tests for analytical purposes are limited to those that produce a strong light in the visible part of the spectrum. Almost all emit energy in the ultraviolet but the eye is not sensitive to that. Ni, Co, and Cu have strong lines approximately 3000 Angstroms which we can't see; however, Cu has one strong line approximately 5700 which is the one we are able to see.