When we conduct a flame test, say we have a solution of $\ce{NaCl}$ mixed with methanol in a ceramic bowl and we light it with a match.

What effect will [Na+] have on the colour we see? If we're assuming that the drop in energy of each Sodium atom (once the solution is atomised) once it gets excited is the same so the wavelength of emission is the same and we see the same yellow colour, if we have a greater concentration of Na+ do we just see more yellow and our eyes would perceive that as being brighter (while the colour itself is the same, we just see more of it)?

Also, I read this on wikipedia:

"Bulk sample emits light too, but its light is not good for analysis. Bulk sample emits light primarily due to motion of the atoms, therefore its spectrum is broad, consisting of a broad range of colors."

I get the idea that if the spectrum of emission is broad, then the colour is not really specific to the ion being tested for, but what I don't get is the explanation for this. What does "emits light primarily due to motion of atoms" mean and why does this phenomena not occur in a smaller sample say a spray of the solution or the sample on the tip of a nichrome wire? Why is it that using a larger sample, which is what I assume bulk sample means like having say 20mL of $\ce{NaCl}$ in a ceramic bowl as opposed to a 1mL spray bottle change with regards to the colour of the flame that we see?

Thanks in advance!

  • 2
    $\begingroup$ Are you aware of the kinetic theory? Basically every molecules possess kinetic energy due to motion. These motions are more evident in large samples with more molecules. Due to collisions, or temperature these molecules may emit photons. These photons do not have a particular value, and may take any value from a fixed range of values. This is a broad range spectrum which poses difficulty in analyzing the color of salt flame. $\endgroup$ – Pritt says Reinstate Monica Jun 4 '17 at 16:05
  • $\begingroup$ Thanks for the response. So essentially, the collisions of atoms in the sample are sufficient to energise some electrons enough to become excited and emit photons and this effect is more prominent in a larger sample? $\endgroup$ – shA3245699 Jun 5 '17 at 8:02
  • $\begingroup$ That may happen, though I'm unsure if thats the majority reason. Please wait for a while until a expert on your topic comes by (I'm no expert). $\endgroup$ – Pritt says Reinstate Monica Jun 5 '17 at 8:59

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