Tubes for Critical Point Demonstrations The construction of tubes using ethane (critical pressure: 48 atm) and trichlarofluoromethane (C.P.:39 atm) has been described by Habgcod.' Since many schools do not have the vacuum line facilities required in handling ethane and triehlorofluorometbane, the elimination of COzcritical tubes (C.P.:73 atm) prevents many teachers from demonstrating critical phenomena at all. We have prepared COZcritical tubes with and without vacuum line techniques by two methods. (I) A test tube end is blown on a 20-cm length of Pyrex capillary (either 1-mm OD x l-mm ID or 8-mm OD x 2mm ID) and the tube is attached to a vacuum line containing a COzreservoir flask. Liquid nitrogen or air is used to condense C02into the capillary. When the capillary is about half-full with solid COz,the system is evacuated and the tube is sealed off. For best results, the tuhe should be about half-filled by liquid COz.If the level af liquid is below this, the Concan be refrozen and the tube shortened. (11) The C02 critical tube can also be constructed without vacuum facilities or liquid nitrogen. The capillary described above is set in a Dewar of Dry Ice and filled with finely powdered Con.The open end of the capillary is drawn, allowed to cool, and then sealed off. We have constructed several such tubes by both procedures; none have exploded. Precautions taken should include: the capillary tuhes are tamh-annealed; the prepared tube is placed in a drawer until it warms to room temperature; testing and use is done behind a safety shield. We demonstrate the critical transition by placing the tuhe, raised slightly a t one end, on the stage of an overhead projector focused on a screen. The temperature is raised and lowered vast the critical temverature by the judicious use of a heat pun and a COzfire extinguisher. X H a b g d , H. W., J. CHEM.EDUC., 33,551 (1956) S. Ruven Smith Ray Boyington University of Connecticut Storrs, 06268
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