MadSci Network: Chemistry
Query:

Re: Why does magnesium glow with such a bright white light?

Date: Mon Jan 22 22:39:54 2001
Posted By: Joseph Weeks, President, Thermal Products, Inc.
Area of science: Chemistry
ID: 979167448.Ch
Message:

What a great question, Sarah.  The subject of combustion is quite complex, 
so, unfortunately, I can't give you a simple answer.  When a metal reacts 
with oxygen, heat is generated and the metal is converted to the metal 
oxide.  The flame temperature is the temperature reached when a substance 
combusts in air with no heat loss.  Not many flame temperatures are found 
on the Internet, but at the site:  http://www.cheater.com/
homework/Homework/Schoolsucks/Uploads/Random/44507017.htm
I found the flame temperature of magnesium to be about 2,800 C or 
3,100 K, which happens to be an exceptionally high temperature.

When a material is heated, it give off radiation; the higher the 
temperature, the more the radiation.  The equation which governs the 
radiation of black bodies is M=sT^4 where M is the The excitance (M) of the 
black body, the power emitted per unit area, s is the Stefan-Boltzmann 
constant and T is the temperature. Thus, we could obtain a twofold increase 
in radiation by merely increasing the flame temperature from, say 2000 K to
 2400 K.  So magnesium oxide which results from the combustion of magnesium 
and oxygen is very, very hot, and therefore, gives off a great deal of 
radiation.  The radiation given off when an object is heated is referred to 
as "black body" radiation.  The equation above must be modified to 
recognize that most materials aren't perfectly black, so they are 
considered "grey body" radiators.  At these high temperatures, however, the 
magnesium oxide is giving off a great deal of radiation.

The other issue involved with black body radiation has to do with the color 
of the light emitted.  A piece of steel heated to 1,000 C glows a bright 
red color; almost no blue light is emitted.  As the steel melts at about 
1,500 C, it gives off a very bright yellow-white light.  The magnesium 
oxide at 2,800 C gives off light which is both much brighter do to the 
increase in black body temperature, and contains much more blue light than 
does the steel in my example.

At http://cc.oulu.fi/
~kempmp/colours.html ,Petri Pihko has provided an 
excellent description of both black body radiation, as well as the type of 
radiation produced by atomic and molecular emitters.  It is interesting to 
note that barium, calcium, and strontium are all involved in the 
characteristic emission of specific colors as molecular emitters.  When 
combined with chlorine, they give off green, orange, and red, respectively. 
 Although they all belong to the same periodic group (they are all alkaline 
earth elements) and react with oxygen, as the molecules become heavier as 
we progress down the periodic chart, the energy released per unit mass 
becomes much less, therefore, the flame temperature lower and the light 
emitted is not so bright.

When various oxides are heated to high temperature, the give off quite a 
bit of light.  In your great grandparent's day, oil lamps heated strips of 
calcium oxide to give off a relatively large about of light.  The calcium 
oxide, often called "lime" lights were popular along the front edge of a 
stage.

So your question involved not only chemistry, but quite a bit of physics 
also.  Keep up the good work.




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