The Use of a Filmstrip Projector to Present a Critical Point Demonstration
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The conceot of critical temnerature and critical nressure is one which manv @ a m. .. . ..... students find difficult to undersinnd.~heautht,rhasiwnd the tollowing drmun, ~ which are awoc~ated stration u, aid students in s,isualinng sume of t h phenomena with the behavim of a fluid at and ahwe thecritival trrnprraturc and pressure. A BLOW. ALLMhJH Pyrex tube, approximately 20 cm in length (9 mrn OD, 3 mm ID), containing liquid carbon dioxide is prepared.' The sealed end is protected by wrapping it with tape and placing a metal or rubber tubing cap over it. To demonstrate the critical point, is removed and the the filmstrio or slide carrier of a standard filmstrio. .nroiector . tuhe isinsekedinto the eavitv. Forsafetv ourooses.aorotectivealuminum shield 1 qp1 1 . . auor + should be custom fashioned to partially -urrvund the tuhe and also tr, f ~ intc t the l' " k"' " ' PYREX cavity uf the projpct8,r used. A front sectional view of one such shleld i + xhown in NBE the aaumpanying figure. The block 1s bwed and the blade is dotted I*,arcornmo' date the tubesothat when theunn is lnsert~dinwiherwity of the pnIJe~wr,ligltt will pass t h n q h the tuhefrom the light svurce u,thescreen.The pmjertor is rhen turned un,and the exit fur theair which couls the bulb ~icortreds o a s t o f ~ r c e t n e hot air onto the tube. The ~roiectedimaxe will be inverted on the screen and it is beneficial to point out the Galis of the tube, the liquid phase, the meniscus, and the gas phase. As the tube warms, the liquid will be observed to hoil and decrease in volume. When the critical ooint is reached (31.1°C. 73 atm.). the meniscus will be seen todisappear as the drns~tyoithrgas hccomrsequal to that ofthp liquid. Thrcover tothe hot direlit isrrrnowdand d forms from the fluid, thr turhulrnce several dmpr tfacetone are permitmi to tlow over thr t u b tocud it. As rhe l ~ q u ~phase and the schlieren effects can be seen in the projrcted image. The heating and ruoling pntcsieq mag be rrpeat~d.W h ~ nnor in use, the tube should be stored in a heavy plastic bottle in a safe location. ~
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' Smith, S. R. and Boyington, R., J. CHEM.EDUC., 51,86(1974). H. LeRoy Nyquist California State University. Northridge Northridge, CA 91330
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Journal of Chemical Education
