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Name: James
Status: other
Age: 60s
Location: N/A
Country: N/A
Date: 8/28/2004


Question:
Someone asked you why Magnesium did not emit white light when subjected to the flame test and

Vince Calder said it will not emit much in the visible region. I cannot reconcile this with the fact that a photo flash is a brilliant white as magnesium burns with the oxygen trapped in the tube. It should be very bright in a flame test; in fact it should almost explode. What?


Replies:
You make a very good point. There are two different processes here.

The first (which was what my assumption was) is that typically a flame test that might be used to identify a metal is performed on a metal salt, usually the chloride or nitrate because they tend to be the most volatile. If you perform this test on a salt such as MgCl2 there will be no color characteristic of Mg because neither the atom nor the cation Mg+2 have an energy level in the visible region of the electromagnetic spectrum.

The second (which is the intense white light) results from the combustion of Mg to produce MgO. Here there is a chemical reaction that produces a large amount of heat energy. In fact, as you point out, the temperature is so high that light spanning the entire visible range of the electromagnetic spectrum is emitted with equal intensity. As a result the combustion flame appears white. That is "just" blackbody radiation at a very high temperature. In contrast, if you were to react the metal sodium with oxygen or air in a similar experiment, the flame would appear as an intense yellow-white flame (explosion) because both sources of radiation-- emission of specific yellow lines and the "white-hot" flame due to the heat generated by the reaction.

Thanks for bringing this distinction to my attention.

Vince Calder


James,

Magnesium burning (as in your flash-bulb example) is a process of combustion wherein magnesium metal is oxidized to magnesium oxide -- an exoenergetic process that does emit a brilliant light. A flame test is done with magnesium ions (the already oxidized magnesium atoms) in solution -- a process that merely involves heating the ions in a flame, a process that does not produce much light in the visible spectrum. The difference is the source of the light -- magnesium atoms burning to magnesium ions vs magnesium ions emitting visible light. By the way, the foil inside a modern flash-bulb is zirconium rather than magnesium.

Regards,
ProfHoff 908


Hi James -

Burning magnesium elemental metal and flame-testing magnesium salt solution are a little more different than you have conceptualized.

A flame test is comprised of:

Getting a metal wire that won't melt in the flame. (Pt?)

Bending a tiny eyelet on the end, so it holds a drop of water.

Putting a water solution with chemicals on that eyelet.

Plunging the eyelet into a hot flame, which preferably doesn't glow very brightly by itself.

Watching for a streak of extra color as the solution quickly evaporates away in the flame.

Any Mg in that water solution is Mg2+ ions; already been burned, so to speak. Certainly need not almost explode.

Burning metal starts with Mg(solid), then vapor of neutral Mg atoms, finally having fumes of Mg2+/O2- solids.

Neutral Mg atoms will have more visible-range transitions than Mg2+ ions.

When an MgO pair forms, the energy is already inside, trying to get out.

So it's very likely to radiate something.

The waste product will be rather concentrated, so it's likely to form a smoke of solid MgO.

Solids usually glow (black-body radiation) more easily than gasses, having more room inside for a variety of energy states. During combustion there may be small particles of unburned metal covered with dark sub-oxides, glowing white hot. (Both sub-oxides and finely divided metals are usually dark. Dark substances glow from heat better than white, clear or silver.)

Flame test, on the other hand, has much less Mg in the area, and it's all Mg2+ ions or MgO monomers, largely dispersed in air as individuals. There are no neutral Mg atoms or condensed metal phases or solid oxides. Single ions will not have darkness or coloration with which to efficiently emit blackbody radiation. They may have few electron transitions in the energy range of the visible light photons, or be immersed in temperature insufficient to excite those transitions. All the energy comes from outside the ion, from a less-brilliant gas flame which is cooler than Mg combustion, and the intent is to see if the flame's lower temperature _can_ excite the Mg+ or MgO so it will re-radiate in the visible. It might be unable, and the ion might never be much excited.

I do not know in any detail how the energy transfers actually work in a flame test, but the answer must be somewhere in what I said. Somebody who does flame atomic emission spectroscopy might know more.

Jim Swenson



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