|
"The Joys of Liquid Nitrogen" (J. Chem. Educ. 1996, 73, 651) describes a fascinating set of demonstrations for children. However I must take issue with the interpretation of "Experiment 3: The Mysterious Liquid Drop". The authors assert that cooling an "empty" test tube in liquid nitrogen results in the formation of a drop of liquid oxygen in the test tube. The drop is not oxygen but a mixture if liquid nitrogen and oxygen, as may be proven by a simple experiment that I have done many times.
After removing the test tube from the liquid nitrogen bath, wait about five seconds and then thrust a burning wood splint into the test tube. The flame will be extinguished. Remove the wood splint while it is still glowing and immediately warm the test tube in a beaker of water
for about two seconds, then thrust the glowing wood splint
back into the test tube. It will reignite.
The logical conclusion is that liquid nitrogen boils
at 2196 °C and its vapor will not support combustion.
Rapid heating causes all of the nitrogen to flash to vapor,
followed by the boiling of oxygen at -183 °C. The oxygen,
being formed second (and being slightly more dense than
nitrogen), remains in the test tube and restores the flame.
I have used this demonstration as part of
a "Dephlogisticated Air Show" I do on occasion at the
Joseph Priestley House in Northumberland, PA, and I believe it
to be my own original.
|
Citation
