tested Jemonstrcrtions A Lemon-Powered Clock Trevor M. Letcher and Aubrey W. Sonemann Rhodes University Grahamstown. South Africa
The modern digital quartz-crystal clock or watch requires a very small electric current for operation. We have measured it a t 1.5 x 10" A at 1.35 V. As a result, the batteries used in these clocks can be small, making a batteryoperated watch possible in recent years. Using this idea, we have successfully powered a digital clock with a number of 'Aomemade" power sources.
edited bv GEORGE L. G I L B E R ~ Denison University Ganville. OH 43023
lemon. Poke the zinc electrode that is connected to the negative end of the battery holder into the other lemon. Finally, take the lead that is not connected to the clock, and poke the zinc end into the lemon that now holds the copper piece. Poke the copper end into the other lemon. You must use two "cells" in series to obtain a potential of a t least 1.2 V. It is best if each "cell" operates in the same segment of the lemon to ensure a good conducting path. The potential we obtained with two lemons was 1.3V. Your configuration should be similar to that in Figures 2 and 3. Your clock should now be operating and waiting to be set! Alternatives
Method Only watches or clocks that use mercury cells (producing a voltage of about 1.35 V) can be successfully used in this experiment because each "cell" of this battery produces 0.6-0.7 V. Remove the "circular disc" battery from a quartz-crystal digital watch or clock, and carefully solder copper leads (about 20 cm long) to the two terminals in the battery compartment (see Fig. 1).If this is done carefully
The two lemons can be replaced by most fruit or vegetables, including potatoes and oranges. You can also use cups or beakers that hold colas or a dilute acid solution (e.g., 0.01 M). Such a clock should operate for weeks without attention to either the electrodes or the electrolyte.
Figure 2.The battery circuit showing the electrodes in the lemons and also connected to a volt meter. Figure 1. The soldered conneaions of the leads to the banery holder. The negative battery pole is connected to the white lead. you can use the watch again after you have completed your experiments! Alternatively, alligator clips can be connected to the watch leads. Solder a copper electrode to the end of the lead connected to the positive end of the battery holder. (A piece measuring 5 cm x 1cm x 1mm cut from a copper sheet is suitable.) Solder a zinc electrode onto the lead connected to the negative end of the battery holder. (Apiece of zinc-coatediron measuring 5 pm x 1cm x 1mm cut from a sheet of galvanized iron is suitable for generating power for a few days or even weeks. Alternatively a galvanized roofmg nail can be used.) Take another copper lead (also about 20 cm in length), and solder a copper electrode to one end and a zinc electrode to the other end. Now take two lemons. Poke the copper electrode that is connected to the positive end of the battery holder into one
Figure 3. The watch is connected to two "lemon'' batteries. Volume 69 Number 2 February 1992
157
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 ~
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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-
