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Heating CaO to create intense light

  1. Nov 9, 2014 #1
    I saw on TV this morning how Thomas Drummond in 1829 created an intense light for use in a lighthouse in 1829, before electric lights. He heated calcium oxide with alcohol and oxygen to a very high temperature creating what is called limelight, later used in theatrical productions in the 19th century.
    What is the chemistry behind this intense light creation by heating CaO?
    The TV said the Calcium Oxide was burnt, but I don't see how that is possible.
     
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  3. Nov 9, 2014 #2

    SteamKing

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    Calcium oxide CaO is made by heating any material containing calcium carbonate CaCO3 to drive off carbon dioxide. The resulting product is called quicklime, and it will eventually react with the carbon dioxide in the atmosphere and gradually turn back into calcium carbonate.

    However, if the quicklime is heated to extremely high temperatures (2400 C), it will give off an intense glow, which is called limelight.

    http://en.wikipedia.org/wiki/Calcium_oxide
     
  4. Nov 9, 2014 #3

    NTW

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    A similar (but more modern) light source is the 'Welsbach mantle' used in street gas lighting until recent times, and still seen in portable gas lamps.
     
  5. Nov 9, 2014 #4

    Borek

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    There is no chemistry behind the light emitted, just physics - everything heated high enough will emit the light.
     
  6. Nov 9, 2014 #5

    NTW

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    ...and metal oxides can have a high melting point. However, there has to be a special reason for using rare metal oxides in the Welsbach mantle. I suppose that it has something to do with the emission spectrum of thorium...
     
  7. Nov 9, 2014 #6
    Well anything heated to a very high temp is going to glow. My room is lit up by a tungsten filament glowing at about 2800 C. Is there anything special about CaO?. Can you use BaO , SrO, or a blob of anything that doesn't melt?
     
  8. Nov 10, 2014 #7

    DrDu

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    As I understand it, there are two reasons why some oxides are more apt to get high luminosity than others.
    The highest thermal emission in a given frequency range does have a black body, which would appear completely black in the frequency range of interest. A grey body emits less radiation when heated.
    Now these oxides have a high emitivity in the visible range and a low one in the infrared (as they are nearly transparent for infrared radiation) so that they get hotter than substances which have high emissions in the IR. I.e. there are less radiation losses in the IR region.
    The other reason why to use rather exotic compounds like cerium oxides is that these oxides catalyze the oxidation reactions, so that heat is directly produced at their surface whence they get much hotter than other substances being brought into a flame.
     
  9. Nov 10, 2014 #8

    NTW

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    A 'black body' in temperatures of the sort we're discussing, 2000-3000 ºC would appear very bright in the frequency range of interest, i.e., in visible wavelengths.
     
  10. Nov 10, 2014 #9

    DrDu

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    Of course. I was referring to it's appearance at room temperature.
     
  11. Jan 22, 2015 #10

    DrDu

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