n
teaching aids
W. Tho Columbus, Ohio ROBERT State Ohiouniversity BARNARD
O n e of the most versatile and valuable devices available for the chemistry classroom is the overhead projector. It can be used in a variety of classroom conditions and is available with a wide number of characteristics. The newest models of overhead projectors give clear images in well-lighted classrooms, are potentially superior substitutes for the chalkboard, and are light enough to be truly called portable. I n addition to handwriting they can project transparencies prepared in advance; motion and color can easily be added; and many demonstrations can be performed on the projector stage such that the true color of solutions or the formation of the uroducts in a reaction is visible to any student in a class~oomwithout straining. The contribution of the overhead ~roiectoris almost exclusively an expression of the teacher. It enables him to do something better than he could without it, yet he completely controls the context, sequence, and equipment (Fig. 1). A
Overheard Projectors are available (see Table 2), hut it should he noted that the shorter focal length lenses tend to introduce distortion at the edge of the picture. Condensing System. There are two principal designs of overhead projectors: Those in which light is reflected out of the light box and into the focusing head by a large parabolic mirror, and those which use a plastic Fresnel lens just beneath the writing stage. Cooling System. If the projector is to be used as a. substitute for the blackboard and will he on for hour-long periods, the tremendous amount of heat generated by the projector's bulb should be taken into consideration. Overhead projectors convert from one half to s, whole kilowatt of power into heat. Thus, the effectiveness of the cooling system is of considerable importance. Some cooling systems have excessive noise and can make it difficult for the instructor to hear questions. Projec-
.
Types of Overhead Projectors
Overhead projectors consist basically of a light source, a light condensing system, a writing stage, and a lens system; there are many combinations available as is indicated in Table 1. Since this projector is an optical instrument, most attention should be given to this area. The following information should be made available to the person selecting a projector for a specific application. Lamp Type and Wattage. Generdly the lamp wattage should not be less than 600 w for a 5-ft square soreen or 1000 w for an aftsquare screen. T h e tungsten-halogen lamps used in many models of overbead projectors maintain excellent light output and give longer life than conventional incandescent lamps. Focal Length of the Prqiedion L a s . A 14in. focal length lens is commonly supplied with the overhead projector. O t h e r lenses
Figure 1. A demonstrotion performed on the rtoge of the overhead with accompanying equations. differences in the two tuber is readily opporenttoitudentr.
Volume 45, Number 5, May 1968
The color
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Table 1. Address
Directory of Manufacturers of Overhead Proiactors
Model
Charles Beseler Co. 219 South 18th St. East Orange, N. J. 07050
Bell B Howell Co. 7100 McCormick Road Chicago, Ill. 60645 Buhl Projector Co., Inc. 1770 New Highwa 11735 Farmingdale, N. l?
Graflex, Inc. Rochester, N. Y. 14603
10 X 10
56027
10 X 10
56013
5X 5
=Master Vu-Graph
10 X 10
1000
14
34
Ewn-O-Graph
10 X 10
1000
14
35
LumsrScrihe
10 X 10
1000
14
23
Porta-Scribe 600
10 X 10
14
17
Porta-Scribe 1000
10 X 10
600 halogen 1000 halogen
14
23
Porta-Scribe Standard
10 X 10
420 halogen
14
17
Has ir filter, lamp changing device. 12'/rin. lens available. Portable version avail. PorteScribe Accessories, Science table, 2 X 2, 3l/rin. slide projection attachment.
301
10 X 10
600
14
17
Has, lamp changing devme.
lOOCF
10 X 10
1000
14
24
*Koolite 6000
10 X 10
1000
14
40
Available with lZ'/riu. or 10L/rin.lens Model 7000 has identical optics hut "low silhouette." All models have removable Fresnel lens for cleanine.
OH-2000
10 X 10
14
22
OH3000
10 X 10
650 halogen 650 halogen
14
24
5 x 5
...
...
... TOPS vertical overhead
14
16
650 halogen
14
17
Built& roll writing attachment.
TOPS Overhead Proieotor.
Ewnomy version VuGraph. Has o~tional12'lrin. ,lens:
projector.
1000
14
...
Travel-Graph 2000
10 X 10
14
17
Travel-Graph 1000
10 X 10
420 halogen 1000 halogen 1000 halogen
14
22
14
22
14
17
14 10'1s
34 10
Uses parabolic mirror.
Science table sttschment. 1000 w projectar a v d able.
Travel-Grauh RXB
10 X 10
600 halogen 7l/* X 10 750 10 X 10 500
Available with 10 X 10in. or 7'/n X 10%. stage. Uses parabolic mirror.
Projection bulb can he repositioned far fine focusing. Has ir filter.
10 X 10
600 halogen
14
15
'Auditorium Overhead 10 X 10 Model 42 Wilson Overhead 10 X 10
Two-1000
14
38
650 halogen
14
161/. Has high impact plastic case.
'Auditorium models suitable for heavy use.
/
600 halogen
Easily wnverts from vertical to horisontd stage for use of TOPS cells.
10 X 10
Overhead Model 66
342
7
"Transpaque Auto Level
~Vizucam Portable Desk-Top Model 88
H. Wilson Cow. 546 W. 119th St. Chicaeo. - . Ill. 60628
5 X 5 f t image from 7-ft projector distance
Remarks
Apallo 6
Travel-Graph Resolute 10 X 10 Technifax COT. Holyoke, Mass. 01040
Weight (lb)
500
Fbytheon Education Co. 285 Columbus Ave. Waltham, Mass. 03154 Projection Optics Co., Inc. 271 11th Ave. East Orange, N. J.
Lamp (w) Lens focal length (in.)
5X 5
AcM)-Matic Stephen A. Lwick Co. 2100 Dremo Road Richmond, Va. 23230 American Optical Co. Buffalo, N. Y. 14215
Writing area.
Journal of Chemicol Edumfion
Table 2. Lens focal, leneth (in.)
40
Screen Size Chart for 10 in.2 Copy Proieotion Distance (ft) Minimum squsra sive screen (in.) 50 60 70 84
86
tors using 1000 w bulbs usually offer ir filters as accessories to further aid cooling.
Additional characteristics of overhead projectors which should he considered before purchasing are: the working height from stage to bottom of lens-so that sufficient room is available for demonstrations; availability of transparent rolls for writing; the light beam adjustment; the projector support; and ease of replacing lamps. Location of the Projector. On a vertical screen, the tilted beam of light from the overhead projector produces an image wide a t the top and narrow at the bottom: very often this "keystone" distortion is no great disadvantage. If the effect is too pronounced, drawings become distorted, and there is excessive reduction of screen illumination across the upper portion of the image. To maintain a suitable angle from the screen to the projection axis, a tilted screen is suggested (Fig. 2).
Figure 2. A number of different screen designs are ovalable for use with overheod projecton lo) A keystone odiurter can b e purchased as an accessory for many tripod mounted screens. lb) Pull downscreens can b e mounted fmm 18 to 24 in. from the wall using simple metal brackets The screen con b e pviled bock at the bottom, e.g. to a chalkrail for minimizing keystaning. k) A sheet of celotex motes an inexpensive projection xreen. Sash cord supports the meon a t the top to adiurt tilt. A 1 X 2 wooden frome cemented to an un-primed celotex foce adds rigidityto thescreen. Mount screens m high or possible.
Making Transparencies
There are several simple methods for making quality transparencies for the overhead projector. The simplest techniques produce black on white transparencies, colored areas, or combinations of the two. The basic transparency is prepared by drawing on 8 X 10-in. sheets of clear plastic available from the suppliers of the projectors (Fig. 3). Additional resources for writing plastic are listed in Table 3; reclaimed X-ray film is the least expensive material; DuPont's Mylar film is a good choice if the intent is to clean and reuse the plastic a number of times. Pencils such as used on the bowling score machines or pencils and pens especially prepared for overhead projection are required. Color can he added manually by means of transparent colored tapes, pencils with a special transparent colored wax, felt tip markers or sheets of colored acetate; the sheets are
Figure 3. The prerr-on type letters and toper provide o convenient way lo add flgvrer or captions to transparency ma3ter3. Specify heat resistant materials for maximum Rexibility in copying from a master.
useful if several large areas are to he emphasized as in a phase diagram. Color Methods. In a second technique, s. separate drawing is prepared on a translucent material such as tracing paper for each color desired on the final transparency. Each piece of art work is plrtced in contact with an appropriate diazo material which is then exposed to ultraviolet light passing through the drawing. Diazo+ensitiaed films produce brilliant colored positive dye images on a transparent film hase when exposed to an ammonia atmosphere. (When the sensitized material is exposed to ultrsr violet light, the diazo molecule is decomposed to a colorless compound which is cleared from the film by ammonia vapor.) The 6lms me members of the ammonia. developing diazotype reproduotion (white print papers) in which a positive original gives a positive print. A group of cola= transparencies can be mounted an a common mount to produce a multicolor effect s t a cost of shout 256 per color. A tri-color foil which combines the colors of red and blue on a single sheet with the option to combine the red and blue to form black bas been developed by the General Aniline and Film Corp. (GAF), Johnson City, N. Y. Instead of using two or three pieces to form an overlay, y, single sheet is exposed twice in a copy machine, once on each side to a different master. A modification of the Diazo process has been produced in the Multicolor Kit No. 6300 by the VwiTyper Corp., 720 Frelinghuysen Ave., Newark, N. J. 07114. The dye colors desired at development are supplied in aerosol cans. After the basic foil has been exposed to ultraviolet light, cotton swabs &remoistened with the appropriate spray and rubbed onto the plastic in the area where the lines or letters are to appear. A number of color combinations in a single sheet are possible; this process has been highly suitable for colored line transparencies of structures, graphs, etc., but lacks even color development for lsrge areas. A single composite master is used with this system.
Table 3. Panspencils proieet as: Color
Suppliers of Overhead Projection Materials Mars Lumacolor
Color
Pmiwto-Color
Color
Sanfords
Bhok
Beseler Vu-Graph marking pencil /7281
Pressure sensitive films, odored or patterned transparent tapes, and presetype transfer lettering
Rwlaimed X-ray film
8%
X
11-in.; 0.010
BsWe
D u Pont polished Mylar. 0.003 Mil thickness. sheets or mils
J. S. Stsedtler Ino. M O ~ ~ V ~ I I . . N. j. Koh-I-Noor Inc. ~ l o o m s b u r N. i J. 08804 Sanfords Beiiwaod Ill. Charles deseler Co, 219 18th St. Esst Omnge, N. J. 07018 Artype, Ino. 345 E. Terra Cotts Ave. Crystal Lake 111. 60014 Chart-Pak lhc. 1 Ri"er &d Leeds. Mass. 01053 Crafting Mfg. Co. 18501 Euehd Ave. Cleveland 12. Ohio SPC Corp. BOX 407 Chatham, N. J. Johnson Prooeas Co. 80-88 Front St. Elirabeth. N. J. Plsatio Pmduots of Utah P.O. Box 1415 Salt Lake City. U b h
S.
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Figure 4. A simple copy mochine con b e conrtrvcted "ring '%in. or the boric material. Material specifications ore not critical; however, only one timer ho. been found to b e suitable: a Mark-Time model 74701 available from Allied Radio, No. 58E 8259.
The terms Diazochrome (a trademark of the Technifax Corp), diaso, foil, or transparency are generdly used to identify any of these base materials avaihble in 8l/%X ll-in. sheets. Diazo materials are sensitive only to ultraviolet light and make excellent msterials for students or office personnel to handle since no special facilities or handling is required. Mechanical Deuices. A simple copy machine which can be constructed and operated in a department office consists of a. platen to hold the copy material and diaso, an ultraviolet source and timer to control exposure (Fig. 4). Concentrated ammonia. in a polyethylene cake container or in a wide month l-gal. reagent jar is all that is necessary for the developing stage. Detailed instructions for the construction of this copy machine are available from the author. Timing is critical only in the stage where the diazo is exposed to the ultraviolet light. averaaine about 2 min. Develovment in the ammonia can run to several minutes with satisfactory results. There are a number of excellent manuals on how to prepare transparencies.' Potentially an office secretary is the resource to develop for preparing these materials within the department as there is much to he said for having this type of skilled help immediately accessible to the instructor. There are several other simple methods for making quality transparencies for the overhead projector using office copy machines. Perhaps the most popular, because of its speed, is a thermographic process developed by the Thermofax division of the 3M Company. Thermofax transparencies cast from 25$ to 456 per sheet depending on the type and will make an instant transparency in any Thermofax office copier. The major limitation in preparing transparencies using the thermographic process is that theoriginal has to he composedof lines containing carbon, and the detail must be large enough to be easily resolved as the fine detail in lines tends to bleed together in this process. A first carbon copy from the typewriter or India Ink drawings are excellent for the thermographic process. (Later, in the discussion of legibility standards, the cautions concerning the use of typewritten materid for projection is disoussed.) Thermofax makes several transparency films which reproduce the original copy as colored lines on s. clear background, black lines against a solid color background, white lines an 8. colored background, or color negative films which show on the screen as colored letters against a. dark background. The instructor will have to decide which of these materials is the most suitable for his aoolication. Our experience has been that the Type 127 which &es a black MORLAN, JOAN E., ''Prepamtion of Inexpen~iveTeaohimg Materials," Chandler Publishing Co., San Francisco, 1963. POWELL,L. S., "A Guide to the Overhead Projector" (Snd ed.), British Association for Commercial and Industrial Education. London, England, 1966. Technifax Corporation, "~iasoehrome Projectuals for Visual Communication," Technifax, Holyoke, Mass., 1962.
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Journal of Chemical Education
line against a clear background is the mast suitable from the standpoint of resolution and ease of the instructor reading the material from the projector stage. Transparency films are now available for the electrostatic wpy process from ArkwrightJnterlaken Corp., Main St., Fiskeville, R. I., and Direct Reproduction Corp., 835 Union Street, Brooklyn, N. Y. 1121.5. This material replaces the paper in the storage drawer of a Xerox machine and transparencies can be made directly from any material which can be copied on the Xerox machine. It is best to have a qualified Xerox serviceman handy the first time the transparency materials are tried; the plastic has been known to jam the feed mechanism in incorrectly adjusted machines. The spirit duplicating process is potentially a useful device in preparing lecture materials if the instructor wishes to give a wpy of a handout to each student and have an identical transparency far project,ian. I t is possible to buy special masters (Transofax type 1250T available from Columbia Ribbon, Glen Cove, N. Y.) so that the master is a projection transparency after the copies have been run. Full color transparencies can he made from clay-based printed matter using the lxminatawatet lift method. Self-adhesive (T. J. Donahue, Corp., Kimhertan, Pa. 19442) or heat sensitive film (Seal, Inc., Derby, Conn. 06418) is applied to the image surface. The bonded material is placed in water and the paper and excess olay iis separated from the plastio. After washing is complete the dried plastic is sprayed with a, transparent plastic spray to increase transparency. This process tends to be somewhat of an art, and since i t destroys the original copy, testing of the process on other than the particular picture to be reproduced is suggested. Legibilitg Standards. One of the reasons that the overhead ~roiectoris such a useful device in the classroom is that it maeni-
seeing all of the material presented on the screen. A rule of thumb is that the students in a back row should be no further than six times the image width from the screen. Establishing a lettering size and line weight standard based on this 6W rule iF suggested. Unfortunately this precludes one of the easiest and most accessible forms of neat letter formins-the tvnewrit,er. . , Pic* type. i < . rcxnmrh i- nheut ime-hlli the u ~ ~ u i ~ u uIwg111 u t and I .I : I i t m i I l-in.) liei lens. The typing from primary typewriters can be used with marginal success; the 24 point adhesive transfer letters, the f240 Leroy lettering, and the Rapidogreph template, number 2 (Fig. 5) provide a reliable standard type size and when used in a 36 point or larger type add variety and legibility to headings. I t is probably best to make transparencies in a horizontal formet (7l/1 X 10 in.) whenever possible so that the projector need
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~~~
~
Figure 5. The Rapidogroph pens and templates supplied by the KohI-Nom Corp. provide an inexpensive woy to produce neat letterng wing drawing ink. Templates ore marked or to sire of pen required.
not be pulled unnecessarily close to the screen, violating the 6W rule, in order to get all the material an a t once. Writing out notes for the lecture in advance has obvious advantages. However instructors must be certain that the notes remain on the screen long enough far students to take notes. Figure 6 illustrates the addition of s. simple ratsting polrtroid disc in front of the projector lens. This can give the effect of motionsuch as vibrating, radiating, travelling, or rotatory-to the material on the slide. The "breathing" motions of wrtter molecules can be illustrated in this way. Technamation, Inc., 16 Sintsink Dr., E., Port Washington, N. Y., produces inexpensive motorized polarizing spinners and the motion Figure 6. Motion attachments for the overhead projector ore ovoilmaterids. able in two model* one with Commercial Transparencies. flxed speed, the other with a variThere me many sources of able speed motor drive for the prepared transparencies, and polomid analyzer. The mimotsd bulletins detailing the con- effects possible in o transparency tents are available from the are more effective generally if the manufacturers (Table 4). animdion speed is adjusted by the for on optimum effect. KeuffelandEsser supplies the instructor Mounting brackets ore ovoilrrble masters for 80 transparen- t o odopt this unit to m y modal cies in book form (set no. overheod projector. 30-6020) which can be produced usina diaeo materials. In addition to basic transparencies the 3M Science Series has developed several combination units of text and transparency origin& for CHEM study and the following Reinhold publications: "Principles of Chemical Equilibrium," Morris; "Cbemical Energy," Strong and Stratton; "Acids, Bases, and Chemistry of the Covalent Bond," VmderWerf; "Chemical Bonding
Table 4.
Sources of Prepared Tronsporencies for the Overheod Projector
W. A. Benjamin, Inc. 1 Park Avenue New York. N. Y. 10010 John Colburn Associates, Inc. 1122 Central Avenue Wilmette, IN. 00091 Keuffei & Esser Co. 300 ~ d ; m s Street Hoboken, N. J. 07030
3M Company Visual Pmdwta
2501 Hudaon Road St. Paul, Mim.55119
Mettler Instrument Cow.
20 Nasasu Street Prinoeton. N. J. 08540
Technifsx Cow. Halyoke. Maw. 01040
snd the Geometry of Molecules," Ryschke&sch; and "Electronic Structure Properties and the Periodic Law," Sisler. Periodic reviews of prepared overhead projection transparencies and expanded references of sources are planned as a. feature in the Teaching Aids column. Dr. S. Schrage, University of Illinois, has consented to handle this feature as well as the periodic f h reviews.
Demonstrations
Demonstrations can be effectively presented on the overhead projector, both on the horizontal stage of the 10 X 10-in. projectors or by using the projector with the stage in the vertical position. Dr. Hubert Alyea's TOPS projectors and Plexiglas cells utilize a vertical, 5 X 5-in. format, and as his unique concept is described monthly in THIS JOURNAL, no other reference should be necessary. Several manufacturers offer science table
accessories which convert the conventional overhead to a vertical configured projector (Table 1) which will accommodate the TOPS cells as well as larger materials. Previous publicationsZ give ideas of how solutions can be handled on the horizontal projector stage. Other demonstrations which can be projected as they are performed include meters, models, ripple tanks. The clear styrene boxes used to package tools, jewelry etc., make excellent containers for water in the ripple tank. Figure 7 suggests a design for a projection Geiger counter meter; the device serves as a holder for
" . m a-
Figure 7. Exploded penpostive of radiotion counter. The type meter will vary according t o the design of the Geiger counter. The meters have a Plexiglws body and face and in thb unit the back of the meter body is sawed away to the back of the dial; the metal d i d i s removed, and the back of the meter replaced with 0 sheet of gloss. A 4-in. meter face war used in the abovedevice.
the GM tube, sample absorbers, and the meter. A lime of meters with Plexiglas bodies which can be converted to projection devices is available from the Ideal Precision Meter Co., 214 Franklin St., Brooklyn 22,
N. Y. Dr. Rod O'Connor, staff associate, Advisory Council on College Chemistry, has devised a number of mechanical analogs and devices using magnet models for the overhead projector and the following are Dr. O'Conuor's descriptions of several of these demonstrations. Magnetic Simulation of Electrostatic Phenomena Strips of magnetic material (available from Leyman Corp., 5178 Crookshauk Road, Cincinnati, Ohio, or Edmund Scientific Co., 101 E. Gloucester Pike, Barrington, N. J.) c m be obtained with one maior face of the striu uolarized N and the other S. Bv attach& such strius in suiL61e fashion to stvrofoam models anb simulate the behavior of vasiaus types of eleotroststic system. Several examples are given below. Neutrmz-Prota Interactions. Cut ten 1-in. diameter cylinders signs in from '/&I. thick sheets of foam polystyrene. Cut the centers of five of the cylinders and secure '/s in. wide magnetic strips to the outsides of these cylinders using thin waterproof plastic tape. Be sure that d l these cylinders have the same external polarization. Wrap the remaining cylinders with iron wire. The cylinders simulate protons and the others simulate neutrons.
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KEENAN,C. W., J. CHEM.EDUC.,35,36 (1958). SLABAUQH, W. H., J. W. H., J. CHEM.EDUC.,28, 579 (1951). SLABAUQH, CHEM.EDUC.,30, 68 (1953). Volume 45, Number 5, May 1968
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Place a large rectangular hakimg dish containing about an inch of water on the stage of a horizontal overhead projector. Float two of the cvlinders so that their shadows are projected onto a screen. Bym&ing one near the other, show that they simulate like charges by repelling each other. cylinders and replace by two of the other type. Remove the By moving one near the other, show that they neither attract nor repel each other and thus simulate uncharged bodies, in this case neitrons. Remove the "neutral" cylinders and replace by the five cylinders. Show that these resist clustering. Point out that no stable nuclei exist consisting only of collections of protons. Now add t h e "neutral" cvlinders. Note that clusterine ocours.
+
+
+
protons (Fig. 8).
Figure 8. (ol "Protons" alone.
Ib) "Protons" plus "neutrons;
Simulatia of Hydroge+Bonding. Cut four models of water molecules from '/An. thick sheets of foam polystyrene. Cut signs in oxygen part of each model and signs in hydrogen part of each model. Tape small pieces of magnetic strips along the most remote edges of each model, using one polarized face out for hydrogen and the other face out for oxygen (Fig. 9). Place a large rectangular baking dish containing an inch of water an the stage of a horizontal overhead projector. Float one model in the dish and then add a second. Note orientation effects as models attract the fourth model. Observe orientation and - signs do not represent "full" effects. Point out that charges, hut only relative polarities. Cut to slide or transparency of 3-D structure in ice. Far a modification of this compare scale models of H B and HIO; compare models of ethanol and dimethyl ether. Simulaiion of Electrophoresis. Cut 10 l-in. diameter cylinders from in. sheets of foam polystyrene. Prepare five as per directions for protons in the first example. Use opposite polarize tion of magnetic strips for the other five and cut - signs in centers of these. Select two powerful bar magnets and determine in advance which end attracts cylinders. Mark that end 'I and the other pole of the second magnet Also preand - signs. pare lsrge cardboard Place a large rectangular baking dish containing about an inch of water on the stage of a horizontal overhead projector and float the cylinders in the dish. On a second projeotor, start a gel electrophoresis separation on a Cu-CnO7system. Slip the cardboard sign under one end of the baking dish and the cardboard - sign under the other end. Clamp the
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+
+
+
+
"+".
"+"
1
Figure 9. lo) Projection xene of model "water molecules." A glass cake plate with bowom just covered with water ollow~the models to move freely without turning over. Ib) Model of woter moleculecut from sheets of foam polystyrene. Strip magnets are glued to the edge using 3 M Spro-ment adhesive or equivalent.
'I+,.
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Journal o f Chemical Education
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pole of one bsr magnet in the baking dish near the sign and the "-"pole of the other magnet in the other end of the dish. Agitate the cylinders gently to break up clusters. Compare this analogy with the projected electrophoretic separation.
Solubility and Liquid-Liquid Extraction Apparatus and Supplies Standard horizontal overhead projector Vertical overhead projector 12 transparent marbles 12 smdl iron nails 12 smdl magnets 3 ml distilled water 3 ml diethyl ether (CAUTION) 20 mg methyl purple &in. Petri dish 10 ml flattened test tube (TOPS) Procedure. Arrange the overhead projectors for simultaneous projection onto two screens or mssk portions of each projector for use on a single screen. Place marbles in Petri dish on horizontal projector. Shake to show that marbles do not attract each other. Place ether in test tube on vertical projector. Slowly add nails to marbles and mix. Show that intimate mixing is easy, even though marbles and nails do not appear to attraot each other. Add methyl purple to ether. Slowly add magnets to marhle-nail mixture. Show that tvo distinct regions form (marbles versus msgnet-nail clusters). Add water to ether-dye mixture, shake thoroughly and replace on projector. Dismsion. Note that one aspect of considering solubility is concerned with interparticle forces. Note that failure of ether to dissolve is not because of strong interparticle forces in ether. Ask for suggestions on how some substances mey be readily extracted from their aqueous solutions by ether.
