overhead projector demon/ tration/ Physical and Chemical Properties Erwln Boschmann Indiana University-Purdue University at Indianapolis Indianapolis, IN 46202 Teaching with t h e overhead projector i s hoth f u n a n d pedagogically sound. Below a r e listed some simple ways t o demonstrate various physical a n d chemical properties of matter. Phvslcal ProDertles Chemistry instructors often forget t h a t demonstrations of ~ h.s j i c a . protwrties l . r a n he iuit as effective as rhrmical d e m . onstrations. H e r e are a few examples: Vapor Pressure. The rate of evaporation of different liquids can he illustrated by simply placing one drop each of water, alcohol, acetone, and ether on separate spots an the overhead projector surface. One bv one the soots disaooear. After several minutes anlv ~, the wnter >pot rrmnins. The effect 131 molrrular wight on vapor pressdrr is raiily demonarrated h y comparing the ewporatlon rates of single drops of pentane, hexane, and heptane. Dissolution. If a sugar cuhe is placed in a large beaker or crystallizing dish containing water about 1 in. deep, the shape of the cuhe gradually loses its sharpness and "fingers" of concentrated sugar solution flow away from the cuhe. The demonstration takes only a minute or two and is very effective. Factors affecting hoth the rate and the extent of solubility can be discussed. Antifoam Agents. Suds formation in soapy water can he reduced dramatically by adding an antifoaming agent, such as a higher molecular weieht alcohol. hlopnrric. Fidd. If a flat magnet is placed on the ovrrhead projertorstagr. covrred wirh R ~learacersteshret,and then sprinkled with irm dust. th? ~ h n n rot the mametic field is ourlined hs the iron dust. Boiling Point Dependence on Pressure. If a sample of hot water is placed in a large transparent syringe (preferably glass), and the needle opening is tightly sealed, pulling an the plunger of the syringe (as it lies across the overhead projector stage) reduces the pressure inside the syringe and causes the water to boil. Low-boiling liquids, such as pentane or ether, can he made to boil very easily at room temperature. Molecular Motion. The motion of gas molecules can he simulated by placing a few glass heads (or pieces of metal shot) in aclear plastic box and using simple hand motion (or a vibrator) to give them random motion. Raising the temperature is simulated by increasing the amount of vibration. As the "molecules" move faster, they hit the walls of the box more often, simulating an increase in pressure. Reducing the volume of the box, by moving inward like a piston a piece of plastic cut to match one end of the boa, results in more frequent impacts against the sides of the box, indicating higher pressure. Using glass beads of two or three different sizes can show the relationship between mass and velocity for the various kinds of "male~ules".~ Crystal Packing. Hexagonal close packing is demonstrated very simply by positioning six coins af the same size around a seventh one. Layering (e.g., to compare face-centered and body-centered ~
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' Alyea, H. N.. private communication.
Kolb. D. J. Chem. Educ. 1987,64.348-351
edited by DORIS KOLB Bradley University Peoria. iL 61625
cubic arrangements) can be illustrated hy using metal or plastic rings for one layer and coins of the same size for the next. Colored disks of clear plastic can also he used to illustrate the layering of atoms in crystals. Crystal Shapes. Actual crystals show up well on the overhead projector, especially if the crystals are fairly large and somewhat translucent. Large calcite crystals show up very well. Diffusion of Gases. Cut a piece of large-diameter (1fAO-rnm) glass tubing about 22 cm long, and fire polish the ends. Fit the tuhe with two corks around which some cotton has been wrapped. Lay the tube across the projector stage on a transparency sheet labeled HCI at one end and NH3 a t the other. Drop concentrated solutions of HCI and NH3 onto the cotton-wrapped corks, and insert them simultaneously into the appropriate ends of the tuhe. A solid deposit of NHICl appears a t the spot where the two gases meet. With the aid of a transparent ruler laid next to the tuhe, it is possible to verify Graham's Law. Chemical Properties A good selection of demonstrations illustrating chemical properties was given b y Kolb2 i n t h e lead article for t h i s series. A few additional ones a r e listed here. Oxidation of Iron. Pour a few milliliters of 3 M nitric acid into a small beaker or Petri dish, and add a little hit of steel wool. The "rusting" of the iron is quite rapid in the presence of the acid. Ionization Energy of Alkali Metals. Very small pieces of metallic Li, Na, and K are dropped into three separate heakers containing water a t least 1em deep (to avoid cracking the glass). A transparencv sheet over the oroiedor staee should identifv the three metals &d indicate their i o n u a t i ~ ne&+. Aromic sbnhuls surrounded by circles ~h~rwing the relativrniresof the rhrce metal atoms helps to explnm why the uuttr electron is more easily removrd from K than from Li. Comparing Thermal and Catalytic Decomposition of Hydrogen Peroxide. Place a drop or two of fresh 6% Hz02in the middle of one half of the overhead projector stage. The heat from the lamp will induce slow decomposition, evidenced by the gradual formation af bubbles of oxygen. On the other half of the projector stage show the more rapid catalytic decomposition. An attention getting technique is to use aelean surgical lancelet to draw a hit of blood from afinger, spreading it thinly over a spot on the projector surface, and then adding a drop or two of H2Oz. Acid Rain. The devastatine " effects of acid rain are shown hv nroinrtine onto the screen a time-seoarated air of ~hotoeraohsof a statue tnkrn het'cmmd after tworion by arid mi". Then a spatula is used ro scrape some chips off a piece of marble tur culcirr, or dolo. mite) right onto the projector surface. A drop of dilute acid causes the particles to bubble and dissolve.
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More Electrolysis Experfments for the Overhead Projector Kenneth E. Kolb Bradley University Peoria. IL 61625
A simple cell made from two lead pen& a n d a 9-Vbattery for electrolysis of aq. K I o n the stage of a n overhead project o r was described i n a recent paper.' A modified cell a n d additional demonstrations for t h e overhead projector a r e Volume 64
Number 10
October 1987
891
connected i n parallel can b e used t o double current flow and, thus, halve t h e time of electrolysis. T h e modified cell remains inexpensive, simple, a n d compact (see figure). T h e following additional demonstrations can b e performed with either cell.
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12'pt
tips
Elecbolysis Assembly
described in this note. T h e original cell is easily modified t o use inert electrodes a n d increase current flow. T h e pencil electrodes can b e replaced with modestly priced platinum electrodes, available for a b o u t $15 a pair (Sargent Welch S-29125 or VWR Scientific 21363.005). Two 9-V batteries
892
Journal of Chemical Education
(1) Electrolysis of 10% aqueous solution of NaBr in a Petri dish, readily produces an orange spot of bromine a t the anode. (Platinum as an electrode has an advantage here, since bromine does attack the lead pencil electrodes.) Several demonstrations can be ~erformedwith the bromine. Addition of a drop of 10%aq KI in the area of the bromine immediately liberatesbrown io&ne, the oresence of which can he confirmed bv addition of a drop of crnrchroiution. R e a c t i m d Immine w i t h organicrompoundaia easily demmrtrared hy adding several drops ol rither nn nqueour solurton of L-lutme-1.J-did !aikme addirian, or phenol (aromatic substitution). (2) Electrolysis of saturated aq NaCl produces copious hubbles of Clz gas at the anode of H1 at the cathode. Again, a drop of aq KI near the area of the C12bubbles results in the immediate appearance of iodine. If a trace of dye (e.g., litmus, methyl viol& is added to the NaCl solution before electrolysis the bleaching action of aqueous chlorine is demonstrated. (3) These cells can also be used to liberate metals a t the cathode by electrolysis of 1M aqueous solutions of their compounds. Perhaps the most striking demonstration is the electrolysis of a SnClzsolution. When carried out in a Petri dish on the overhead projector, thin needles of crystalline tin can be seen growing quickly in a wehlike pattern. (If the aqueous SnCIz solution is hazy, a drop or two of eonc. HCI will clear it up.) (4) Electrolysis of 1 M solutions of CuClz and AgN03 also provide visible crystal growth. However, the crystal patterns from Pb(NO& and ZnClz are unique and more interesting. Electrolysis of lead nitrate solution gives a rapidly forming web of fine threads, while electrolysis of zinc chloride yields a solid mass of dendrites in the shape of a growing shell.
Work done at the Institute for Chemical Education at University of Wisconsin, Madison. Kolb, K. E.; Kolb, D. K. J. Chem. Educ, 1986, 63,517.
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