Properties of Air-A
Freshman Chemistry Lecture Demonstration
Commonly, there is a point in freshman chemistry courses in which the topics of oxygen chemistry, combustion, gas properties and rates of reaction are discussed in fairly close proximity. A simple and yet dramatic lecture demonstration can be used to help tie all of these concepts together. A half pint Dewar flask is partially filled with liquid nitrogen. If an ordinary test tube, several inches taller than the Dewar, is placed in this flask, a milliliter of liquid air will condense in the tuhe after several minutes. When the tube is removed, the liquid air is quite visible. The tube will immediately begin to warm, vaporizing the liquid air. A separation of the major components will be effected during heating since nitrogen (h.p. -196'C) is more volatile than oxygen (b.p. -183°C). As a consequence, the gases immediately being displaced are rich in nitrogen, while the last are quite rich in oxygen. A burning splint introduced into the neck of the test tuhe soon after it has been removed from the Dewar will be extinguished because of the lack of oxygen. During the seconds required to explain this observation, the last traces of the liquid air are disappearing. A glowing splint now introduced in the neck1 of the test tube will burst into flame because of the high concentration of oxygen gas.
The glowing splint should not be immersedin the liquid oxygen. To do so risks an explosion,
Glassboro State College Glassboro, New Jersey 08028
Volume 51. Number 11. November 1974
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