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?
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.
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.
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
When an MgO pair forms, the energy is already inside, trying to
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
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
but the answer must be somewhere in what I said.
Somebody who does flame atomic emission spectroscopy might know more.
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Update: June 2012