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overhead projector demon/tration/
edited by DORIS KOLB Bradley University Peoria, IL 61625
Introduction to Overhead Projector Demonstrations Doris Kolb Bradley University, Peorla, IL 61625 Most chemistry classrooms are equipped with overhead projectors, hut not all teachers take full advantage of this useful piece of equipment. Nobel Laureate Roald Hoffmann recently commented on the important role that the overhead projector has played in modern education all over the world, remarkine that its imoact has been ereater even than that of televisio< ~hemistr; teachers have a special reason to appreciate this valuable teaching tool because the overhead projector provides one of the easiest, quickest, cheapest, and most highly visihle ways to do chemical demonstrations. I t is true that most traditional chemistry demonstrations (especiallv ex~losions!)are not suitable for this mode of presentation; hui there are many worthwhile d e m o n s t r a t k s that are. Projecting live chemical demonstrations onto a screen is hardly a new idea. Sixty years ago Harriett Fillinger ( I ) had already started using an opaque projector to show her students some simple chemical demonstrations. Then in 1940 Oscar Richards (2)described various chemical reactions that could he demonstrated on a vertical projector called a "Delineascope", a forerunner of today's common classroom prnjector. He pointed out that "with this projector transparent, semi-transoarent. or translucent materials. and other materials whichare meaningful in silhouette may he shown on the screen", and he stressed how convenient and effective the mcthud was for showing rhemical demonstrations. In 1951 Wendell Slahaurh (3) reiterated these ideas, urging teachers to consider the-overhead projector for simpie classroom demonstrations and offering a list of specific suggestions for suitable demonstrations. A rather comprehensive volume on "The Screen Projection of Chemical Experiments" was published in 1953 by Ernest Hartung ( 4 ) at the University of Melbourne. He stresses the clear visibilitv of demonstrations that are maenified and projected onto a large screen. His discussion includes many different kinds of optical projectors. About half of the hook is devoted to descriptions of the various types of projection equipment. Although the latter half of the hook describes several hundred demonstrations suitable for optical projection, most of them require projectors other than the common "overhead". A giant contribution to demonstrations by optical projection has been made by Hubert Alyea, Princeton's dynamic master of chemical demonstrations. His spectacular repertoire includes manv.laree-scale demonstrations. hut lone aeo he recognized the advantages of live demonstrations projected onto a screen. They require solittle in the way of chemical materials, and yet they are extremely visihle to the audience. His TOPS Demonstrations column first appeared in this Journal in January 1962, and it ran for nine years. There are several convenient volumes (5.6)that describe manv-e x ~ .e r i ments that can he done using the TOPS projector.
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348
Journal of Chemical Education
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I t is fun to watch Alyea doing the TOPS demonstrations. In order to do them yourself, however, you need a special TOPS projector, or a t least an adapter that will convert an ordinary overhead projector for TOPS use. Unfortunately most chemistry teachers do not have access t o a TOPS projector, and those who do often keep the projector stored, so that they must move it into the classroom each time they want to use it. What makes overhead projector demonstrations ao simple is that almost every classroom has a projector already set up and ready to use. Doing a demonstration may require nothing more than picking up a beaker and a couple of little bottles on your way to class. An entire demonstration often can be carricd in one hand, or tucked into a pocket.
About the Feature
This new feature will appear in fhe Journal periadlcally. describing ~ l a ~ s r o odemonstrations m using lhe overhead projector. The emphasis will be on simplicity. All the demonshations will be easy to prepare and simple to perlofarm.Your own contributions are Invited and encouraged.
About the Editw
Doris Kolb teaches general chemistry courses at Bradley University. She received a BS degree from University of L o ~ i ~ v i land l e MS and PhD degrees fromthe Ohio State University. She has been an lndushial chemist at Standard Oil Co. (Indiana), now Amom 011 CO., and a television lecturer in a PBS serles titled "SpotligM on Research". She was a professwof chemistty at Caning Community College (Corning. NY) and Illinois Central College (East Peoria. IL) before coming to Bradlev. She has also bean a visitina orofessor at the Universiw of Wisconsin in adi is on and an instructor in the summer programs of the Dreytus Institute and the Institute for Chemicel Education.
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Belno Prepared for Overhead Demondratlons The secret of making these overhead den~onstrationsalmust effortless is to set up a tiny "stockroom" in some convenienr lace. You can store it in a small hox. on a shelf. or in a drawer. T h e important thing is t o have'it close a t hand, o e r h a. ~ srieht in vour office. or a t least verv close to it. A suggested "starter" l i d of materials is given in the tahle. Hv tisinesmall d r o o ~ e bottles r for liauid reaeentsand tinv vials for solids, it is possible t o get all the chemicals needed for dozens of demonstrations into a small suitcase. (I use a "train case" that measures 9 X 14 X 7 in.) ~ o s t o the f glassware needed for several dozen demonstrations, such a s those listed below, can he fitted into a canvas hag ahout 10 in. in diameter. I have taken both bags with m e on airplanes a s "carry on" luggage. I remember the first time I ever saw a demonstration on the overhead nroiector. It was durine a conference a t Vanderhilt ~ n i v e r s i t ; almost 20 years ago. Clark Bricker from the Universitv of Kansas added a little ammonium hvdroxide to a ferricchloride solution in a beaker on the stage of a n overhead projector. I was so impressed by the clarity, beauty, and simplicity of the demonstration t h a t I have been doing demonstrations on t h e overhead projector ever since. Doing chemical demonstrations on the overhead projector is certainly quick and easy. It is also very inexpensive because such small amounts of chemicals are needed. B u t the most important thing is t h a t the demonstrations are effective. Students can really see what is happening, even those sitting way hack in the last row. Here are some special rules regarding overhead projector demonstrations: 1. Solutions that are deeply colored should not be more than a few millimeters deep. If a beaker of colored solution appears too dark
on the screen, just pour out some of the liquid and it will hecome
Materials Needed for Dolng All Demonstrations Described
lighter. (Adding more water to the solution will make it more dilute, but the thickness of the solution will increase simultaneously, so that the color intensity will not change.) 2. Reactions that give brightly colored precipitates may produce onlv a black soot on the oraiector . . screen. Unless .oreeioitates . are very slow in forming or gelatinous in nature, they may not be suitable for overhead projection. 3. Beakers and other containers should be placed as close to the center of the projector stage as possible in order to minimize the distortion due to parallax. 4. Small dropper bottles provide the easiest way to add reagents. Often a drop or two is all that is needed. A dropperful of liquid (theamount that ispickedupand delivered by a typical dropper) is about 0.6 mL. Some Demondratlons llludratlng Basic Chemlcal Concepts All of the following demonstrations are carried out on the liehted staee of an overhead ~ r o i e c t o r T . h e tahle summarizes the materials needed t o all of these demonstrations. Activity Series or Reaction Rate To three beakers containing dilute acid (6 M HCI) about 5 mm deep add strips of (1)copper, (2) zinc, and (3) magnesium. Hydrogen is evolved (1)not at all, (2) slowly, and (3) rapidly. Catalysis (Heterogeneous) Onto a thin layer d m u t R m m deep) of Job hgdnxgm peroxide in a f'rrri di4n or brakrr drup a small amount uf rilvrr vxiue from a spntula. Iluhhles 01 orygrn torn, on rllr cdrdlyrt surfdrv.
Another substance, such as MnOl, can also he used as the catalyst. Catalysis (Homogeneous) To several milliliters of 0.01 M KMnOn in a beaker add a dropperful of saturated oxalic acid and a dropperful of 6 M HzSOa.The color will be purple. 2 Mn0,-
+ 5 C,O>- + 16 Ht
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10 CO,
+ 2 Mnzt + 8 H,O
l"..."l*\ HalfOunce Square Dropper Bonlss
Tweounce ArnLwCroppereonIes
Ammonium thiocyanate. 3 % Chlorine water Cobalt chloride. 2% Oimethylglyaxime. 0.5% in EtOH Hydrochloric acid. 0.1 M Hydrochloric acid. 0.01 M Mercury (11) chloride. 1% Nickel nitrate. 2% Oxaiie acid, setinrated POtaSSium dichmmate. 2 % Potassium lerrocyanide, 2% Sodium hydroxide, 0.1 M Sodium dihydrogen phosphate. 2 %
Acetone Ammonium hydroxide, 6 M Copper sulfate. 2% Hydrochloric acid. B M Hydrogen peroxide iron (ill) chloride, 1% Manganese (11) sulfate. 1 % Potassium chromate, 4% Potassium permanganate. 0.01 M Sodium hydroxide, 6 M Sodium sulllte. 2% Sulfurlc acid. 6 M
(Indicators) Bromthymol blue. 0.1% In 20% EtOH Congo red. 0.1 % (aq) Methyl orange, 0.1 % (aq) Methyl red. 0.1% in 60% EtOH Methyl violet. 0.1% (aq) Phenol red. 0.1 % in 20% EtOH Phenolphthalein. 0.1% in 60% EtOH Thymal blue, 0.1 % in 20% EtOH
Aluminum nilrate Pmassium carbonate Potassium iodide Silver oxide Sodium chiaide
Solids in Small VlalrP
Metal Strips (about 6 cm long)' COPPBCMagnesium. Zinc
Four-Ounce Plastlc Wash BonleJ.
Equipment
Distilled water Hydrochloric acid. 1 M Sodium hydroxide. 1 M
Beakers. Petri dishes Crystallizing dishes Spatulas. Stirring rods Vew strono maanet
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*TIw wash bottles will n d leak It me tips we owered wilh rubber policemen. 'The filledcapsvlesprepareOlame persmsgnetlsmdemonoflation can bestored in a am11 envelope and used over and over again. 'ma W I I aodiummets~ ~ torthehydrogendsmonswatim&,~d bekeptin
in me ~tockrarm.
A few drops of MnSOr solution added to the mixture catalyzes the reaction shown above, causing the purple color to disappear. This is an autoeotolvtie reaction, since one of the nraduets. Mn2+ion. acts as a catalyst. Equ;libr;um, 1 Place a little KzCr04solution in a beaker or crystallizing dish. Using a small plastic wash battle (or a dropper battle), add a little 1 M HCl. 2 Cr0:(yellow)
+ 2 H+ +Cr20?- + H20 (orange)
The equilibrium shifts to the right, and the solution turns orange. Then add a little 1 M NaOH. The OH- ion removes H+ ion, shifting the equilibrium to the left, and the solution turns yellow again. Equillbrium, 2 In a 50-mL beaker mix a dropperful of FeC13 (I%),a dropperful of NH&NS (3%),and about 30 mL of water. The solution should look like tea. (It can be made up in larger quantities and stored until needed.) Pour a few milliliters of the mixture into each of five 50-mL beakers. Place them on a prepared transparency sheet as shown on the tap of the next page. The beaker at the top serves as a color reference. Add a few drops of FeClr, NH4SCN,HgCI2,and NazHPOI to the other four beakers. as indicated. The first two shift the equilibrium to the rtght. and the cohr ilnrkene. The Hg'. removes S C ' Y iun,and the PO,'- removes Fe3*ion, ihifung the equht,rium tv tl,~. left in eachcaic, and the udur l~ghtenl. Transition Metals or Complex Ions (Copper) To a little CuSOl solution in a beaker add a few drops of 6 M NHdOH. A light blue precipitate of CU(OH)~ forms first, blocking out the light. Addition of more NH40Hdissolves the precipitate and produces the beautiful royal blue CU(NH&~+ complex. Volume 64
Number 4
April 1987
349
Alkali Metals or Hydrogen
(deep red)
PO
CNS-
:3-
If you do any demonstrations on the overhead projector, you probably do this one. This was one of the overhead demonstrations described by Slabaugh (3) in 1951. Into a large beaker (about 1 L) containine water about 1cm deeo add a few d r o ~ of s ~henolphtha-
The heat of the reaction melts the sodium, so that i t becomes a molten sphere, and it leaves a trail of pink (because of the OH- ion heine eenerated) as it darts across the water. (CAUTION: It is
Transition Metal Ions or Chelate~(Nickel) T o a thin layer of NiSOl solution in a beaker add a little dimethylglyoxime solution. Although the cherry red chelate produced is a solid precipitate, it coagulates slowly enough t o be visible on the overhead projector.
Transition Metal lons (Iron) Using three small beakers or a three-section Petri dish pour out three small samples of FeCI3solution. To the first add a few drops of K4Fe(CN)esolution. A deep blue color results. To the second add a few drops of NH&NS solution. A dark red color is produced. To the third sample add a few drops of 6 M NHaOH. A brown precipitate of Fe(OH)3 forms, but it is gelatinous and quite translucent. When shown together these three reactions demonstrate the colorful nature of iron compounds.
Acid-Base Indicators, I There are no chemical demonstrations more ideally suited for overbead projection than those involving acid-base indicators. Expensive indicators become affordable when used on such a small scale, and nocompoundsare mare colorful. One way toshow thevariety of colors exhibited by acid-base indicators is to use two Felsen (quartered) Petri dishes. Pour a little 1 M HCI into the four sections of one dishand 1M NaOH intoeachseetion of theother. Add 1drop of indicator to corresponding quarters of the two dishes. Use any four indicators for the four sections (e.g., Congo red, methyl orange, bromthymol blue, and phenol red). Small beakers arranged in two rows can be used instead of the quartered Petri dishes.
Transition Metal lons or Oxidation (Chromium) A common test for chromium ion in qualitative analysis courses involves oxidation t o chromate, followed by further oxidation to perchromate using hydrogen peroxide. The latter oxidation step is shown in this demonstration. Starting with a small amount of K2Cr04in a Petri dish, add a dropperful of 6 M HCI, swirl t o mix, and then add a few drops of 3%H202.The blue-black perchromate shows up well against the yellow chromate solution.
Transltion Metal lons or Complexes (CobaM) T o a small sample of aqueous CoC12 add a little NH&NS solution and thenadd acetone until the mixture turns blue. Thepinkactahedral C O ( H ~ O ) ~ ion ~ +is converted t o t h e blue tetrahedral C O ( C N S ) ~complex. ~(Acetone is a more convenient nonaqueaus solvent than ether, which is often used for this test in the laborato-
Acid-Base Indicators, 2 T o show that some indieatam exhibit more than one color change, use an indicator such as thymol hlue. Into three small beakers pour about 5 mLof 1 M NaOH,distilled water, and 1MHC1, respectively. Adding a drop of thymol blue solution to each beaker turns the solutions blue, yellow, and red. (Thymol blue changes from red t o yellow around pH 2 and from yellow to bluearound pH9.) Thesame demonstration can also be carried out with metacresal purple or cresol red (which change from red to yellow around pH 1-2 and from yellow to purple around pH 8).
Acid-Base Indicators, 3
ry.)
Oxidation-Reduction or Transition Metal lons (Chromium) T o a litrle KICr.O, d l t i o n in a lrrskrr add a rlr
