If other dissimilar metals are used, care must be taken
to ensure correct polarity Connections to the watch in the wrong polarity could damage the quartz crystal. For example if magnesium ribbon and iron are used, the magnesium must be connected to the negative end, and the iron to the positive end of the battery terminal. Theory The cell reactions are
Z? zn2++ 2e
cathode:
Zn
anode:
2W + 2e
Z? Hz
Sample Tube
The EMF of the reaction is given by
Lens
Screen
Optical setup for projecting the image of the tube onto a large screen. The focal length of the lens is about 35 cm. A prism is used to invert the image of the tube so that it appears upright on the screen. The Cu electrode is actually involved only in collecting electrons. It could be replaced with platinum or another inert metal.
A Lecture Demonstration of the Critical Phenomenon Raymond Chang and James F. skinner' Williams College Williamstown. MA 01267
The critical phenomenon is a fascinating topic that is discussed in general chemistry and physical chemistry courses. In recent years, demonstrations of this phenomenon usin various substances have been reported in this JournalJ3 we wish to describe a novel demonstration that is easy to perform and suitable for a large audience. The system used in our demonstration is sulfur hexafluoride whose critical point data are T, = 45.5 *C and PC= 37.6 atm.4 For a Large Lecture Hall With the arrangement described below, an entire lecture class can readily follow, in a partially darkened room, the initial heating of the liquid, the attainment of the critical state, and the critical opalescence that occurs when the tube is allowed to cool in air. The sample tube is prepared by distilling liquid SF6from a lecture bottle into a thick-walled tube (diameter: 12 mm; length: 100 mm) under vacuum using a liquid nitrogen cold trap. About 3-4 mL of SF6 is needed for the experiment. The tube should be a t least one-third filled with the liquid. The tube is then sealed from the vacuum line using an oxygedmethane torch. The figure shows the optical system used for the demonstration. The setup is located at the back of the lecture hall. With a suitabie condensing lens, an imageofthe same tube about 4 ft in height can be projected onto a screen some 40-50 ft away ~ e c a u s ethe-critical temperature of SF6 is rather low, the liquid can be heated easily using a hot-air gun or a hairdryer. For a Smaller Group In a different setup, which is more suitable for a smaller class (about 50 students or fewer), the sample tube at-
'
Deceased 2Smith. S.R.; Boyington. R . J. Chem. Ed. 1974. 51. 86. 3Marzzacco, C. J. J. Chem. Ed. 1986, 63, 436. 4MacCormack K. E.; Schneider, W . G. Can. J. Chem. 1951, 29, 699. 158
Journal of Chemical Education
tached to a ring stand is placed on the lecture bench. A low-power He-Ne laser (Spectra Physics) is aligned so that its red light shines through the tube and is reflected by a small white screen or a large sheet of blotting paper. The sample is first heated above its critical point and then allowed to cool gradually. As the svstem a~oroachesthe critical tem~erature.lieht scatterin~becom¬iceable inside the tu6ing. he' spatterine phenomenon also shows UD as fluctuatine red lieht on th; screen. By adjusting the position of the beam t o j k t above the soot where the meniscus of the liquid would appear, the &tical opalescence can be dramatieally shown iy the, sudden appearance of an intense red glow within the tube when the temperature drops just below To. At the same instant, the red spot on the screen disappears. Caution: Two elass shields should be nlaced-one between the sample tub;: snd the audienrr nnd'another between the ruhc and thr instmrtvr. The laser beam should r u n parallel to the lenph of the l ~ c t u r shmeh, and it should never be directed at the audience ~
~~~~
Do "Likes Dissolve Likes"? An Illustration of Polar and Nonpolar Solvents Wilbur Bergquist BSCS 830 N. Tejon, Suite 405 Colorado Springs,CO 80903
Many science teachers use the very simple phrase, "likes dissolve likes", but hon oflen do students fully understand what is implied about this relation between solute and solvent? Here is a simple and colotiul demonstration to help illustrate the nature of polar and nonpolar solvents. Materials
Iodine crystals (68large crystals) 250 mL water 100 mL ethvl alcohol (1)separatory funnel (I)long-stem funnel (2) 100-mLgraduated cylinders (1)250-mL beaker procedure After identifying iodine a s a nonpolar material and water as a polar compound, place three to four iodine crystals in each graduated cylinder. Add 100 mL of water to one and 100 mL of alcohol to the other cylinder. The appear-
