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Flame colors - where's the chlorine?

  1. Aug 18, 2015 #1
    I'm writing a lab for high school students on the emission/absorption spectrum of table salt as part of a larger project to introduce them to quantum mechanics. I'm aware that there are many such lab instructions already, but that's not the point.

    While doing some tests and writing the lab instructions, I bumped into some questions that a student might also ask occasionally (one always hopes to have such students).

    The spectrum of an ion is different from the spectrum of an atom (I think). When sprinkling table salt into a blue flame, it lights up yellow. This is the same yellow from (atomic) sodium low pressure gas lamps. How does an ionic compound produce an atomic spectrum? I could think of this mechanism:

    The salt crystal splits into separate ions, becoming sort of a plasma in the flame, the sodium acquires an electron from the chlorine, takes up heat energy to go to an excited state and emits.

    There's definitely a lot of salt residue on the table after sprinkling, so I'm not too sure about the salt crystal disintegrating. As an alternative, maybe the sodium produces an atomic spectrum because it shares an electron with the chlorine. But that would need the assumption that an atomic and molecular spectrum have (partially) the same wavelengths. I don't think that's true.

    So what is the exact mechanism/proces happening in the flame? Any or none of the above? I can't think up a quick (table top) experiment to test it, but would like to know.

    Finally, these flame test experiments are of course well known and the flame color is associated with the metallic ion in the salt. Most of the commonly used salts contain chlorine. Why do we only see the color of the metallic ions but no contribution from the chlorine? (maybe it's there, but is drowned in intensity by the metals, I haven't been able to check with a spectroscope yet).

    The lab can be done without addressing these issues, and that's probably a good idea to not overly confuse the majority of the students. Still, I'd like to include the full mechanism in the teacher's guide.
     
  2. jcsd
  3. Aug 18, 2015 #2

    CWatters

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  4. Aug 18, 2015 #3
    Thanks CWaters. Looks like the explanation I had thought of was pretty close :) Any other references or notes?
     
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