0 Replies Latest reply on Sep 9, 2014 5:47 AM by Andres Tretiakov

    Burning diamond and graphite in liquid oxygen

    Andres Tretiakov

      The first part of the video shows how liquid oxygen (LOX) was prepared by passing oxygen gas through a coil of copper tubing immersed in a Dewar flask filled with liquid nitrogen at -196 degrees Celsius (77 K). LOX has a boiling of -183 degrees Celsius (90 K), which is higher than that of liq. N2 so oxygen gas condenses into a beautiful pale blue liquid.

      As LOX was collected in an unsilvered glass Dewar, the colour was really appreciated and its paramagnetic property (attracted to a magnetic field) demonstrated by using a powerful neodymium magnet. Molecular Orbital theory can be introduced to show that the reason why LOX is pale blue and paramagnetic is because it has two unpaired electrons.

      Next, a small natural rough diamond was heated red hot with a butane torch and dropped into the LOX where it continued to react with oxygen releasing more energy in the form of heat floating around until it completely disappeared. The diamond reacted with the excess 100 % oxygen to form carbon dioxide (complete combustion).

      Diamond is one of the allotropes of the element carbon, together with graphite, fullerenes and lately graphene which has a 2D crystalline structure. In 1772 the French scientist Antoine Lavoisier was the first person to burn a diamond in a closed glass jar by focusing the rays of the sun on the diamond with an enormous magnifying lens. He noticed that the diamond disappeared but the overall weight of the jar did not changed. Swedish chemist Carl Scheele also demonstrated that graphite reacted with oxygen to form carbon dioxide and concluded that it must be another form of carbon. But it was the British scientist Smithson Tennant in 1796who deduced that diamond was another pure form of carbon. By burning equal weights of charcoal and diamonds he obtained the same amount of carbon dioxide.

      A rather energetic reaction was observed when a larger piece of graphite was heated to red hot with a torch and dropped into the LOX. The bright and intense light was similar to that produced in a carbon-arc lamp. Finally, a digestive biscuit was soaked in LOX for a few seconds and set alight with the torch. The impressive yellow/orange flames were caused moly by the carbon from the carbohydrates in the biscuit burning and also due to the excited sodium atoms present in the salt (sodium chloride) added to the biscuit.    

      WARNING: LOX is one of the most powerful oxidizing agents known (used as rocket fuel in the Space Shuttle main engine).

      Organic material will burn rapidly and might even detonate in contact with LOX if any sources of ignition (sparks, flames, static discharge or even impact) are present. 


      REFERENCES:
      http://www.chemicool.com/elements/carbon.html

      An excellent video with Professor Al-Khalili from CHEMISTRY A Volatile History showing a diamond burning in LOX:
      https://www.youtube.com/watch?v=mzah-TEBaF8