J. A. Campbell'
Harvey Mudd College Claremont, California 9171 1
The fact that scientists deal with real, reproducible systems allows them a great advantage over teachers in other s u b j e c t ~ t h e ycan bring real systems into the classroom, then experiment with and discuss the reproducibility until students can tie direct observation and satisfactory comprehension together. Yet little classroom experimentation is actually performed; less than 10% of all chemistry teachers do any lecture experiments at all. The reasons are manifold but the rate determining step seems to be setting up simple, clear experiments readily observable by large groups of students. I n an attempt to change the rate of this step we have collected a set of sealed tube experiments. Our sealed tube experiments must meet the following criteria 1) Sebup and clean-up times of 1es than five minutes and usually less than one minute. 2) Unexpected and dramatic results observable by a large class. 3) Experimental times of five minutes or less. 4) Simple of results in terms of importallt enereet,ic. and structural conceots. , dvnamic. "
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A few of these experiments were invented by us, more were adapted from others. Those we find useflll, regardless of our own contribution to them, are listed below for greater completeness and accessibility for teachers interested in performing lecture experiments. Most have been available for sears, but oerhaas the organization and emphases outknedhere will irkease their usefulness and use. Let the emphasis be clear. These are intended as experiments to be studied and interpreted by the students, not demonstrations to "support" already presented theories. Their effectiveness is greatest if the class is surprised a t the observations and then delighted at the simplicity of the interpretations. Every experiment for students should have these features if i t is to have maximum teaching effect. The experiments are listed in two ways: (a) by fields with multiple references to individual experiments, and (b) individually with directions for performing them. The debt to past teachers is great but no attempt is made to identify the sources of the experiments. Those we have found most useful are indicated by a double asterisk (**), those slightly less so by a single one (*), but all are useful under appropriate circumstances. The cross references in the table, and the descriptions in the experimental section, are not exhaustive and many experiments can be used to illustrate points other than those discussed.
' Present address: Division of Science Teaching, UNESCO, Peris 7. France.
Sealed Tube Experiments
Some sealed tubes. I, evacuated (or unsealed); 11, manometer; Ill, thermmater well (or unsealed); IV, piston or syringe; V, cryophorusi VI, electrode; VII, hollow cane; VIII, U-tube lor sealed).
The "sealed tubes" are not all actually sealed and are classified into one of eight types as shown in Figure 1. They are: I. Simple sealed tube, 11. manometer, 111. thermometer well, IV. piston or syringe, V. crvoohorus. VI. electrode, VII. hollow cane, and VIII." U-tube.' The appropAate Roman numeral is listed with each experiment. Some have more v e cialized construction. They are marked (special). Most tubes are 25 mm Pyrex, but a few are 10 mm. Lower case Roman numerals indicate the smaller tubes. Larger or smaller tubes are possible, of course, depending on class size and the type of projection facilities to be used, Effects
Science courses contain increasing references to thermodynamics and, especially, to interpretations based on the second law of thermodynamics. It is surprising (and indeed shocking) to find that entropy is not mentioned in the experiments in most laboratory manuals, including those designed to be used in courses in physical chemistry. These tubes provide numerous simple examples to attempt to fill this serious gap. Since most are closed systems whose changes occur at essentially constant pressure and constant temperature, the relationship AG = AH - TAS is directly applicable. I n most tubes this equation is equivalent to
Most are reversible so that A S = q,,,/T = k In (W~/WI) are equally useful. Our experience has been that students learn to use thermodynamics intelligently and with discrimination through these experiments and, as a result, we never discuss the Carnot cycle in our courses except in advanced thermodynamics. It is, of course, essential that the distinctions between isolated, closed, and open systems be clearly in hand before entropy changes are discussed. Volume 47, Number 4, April 1970
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Some Subiects Which Can Be Discusre!d in Terms a f Sealed Tube Experimentr~ Kinetic theory. HE and blue glass 23; Sublimation 4. 20 Sulfur, liquid 48: triple point 18 H*. C o t , air diffusion 45 Supersaturation 11 Lamp bulb 32, 50 Le Chateiier's prinoiple-see equilibria Surface enerav 5 Suspension 6 0 Liesegsng rings 46 System Liquid orystals 24 dosed-most tubea Melting point 7, 12, 13, 14 isolated-Dewar flask, stoppered Metals 7, 14. 57 open-beaker Temperature, boiling 17; critical Molar volumea 61. 62 22; triple 7. 18, 30 Moieouiar weight 12 Temperature eReot in equilibrium Nonvoialiie solute 17 35.36. 39.40 Nuoleation 4, 5 Three component systems 27 optieai rotation 59 Thermal efficiency 15 Ores 64 Thermal transpiration 47 r-n interactions 17 Paramagnetism 58 Thermometer. gas 44 Phase rule (see also equilibria) 13. Thermometry, fixed point 7. 18, 29, 24. 26, 29, 30 30 Photo rehotion 38 Triple point 7, 18, 30 Pressure, atmospheric 42, 43 Tyndall effect GO Pressure effect on equilibrium 35 Vaeuum 41 Qualitative anhlsais tests 63 Yen der Wsals forces 17. 22 Radiometer 47 Vapor presaure, orystal 4 Rate of orystallizatian 31 liquid 17. 20. 21 evaporation 4. 21 iowermg 17 reaotion 33, 38 Reversible reaction 2. 3, 38 Vaporization 23 Rhythrnio precipitation 46 Viscosity 48. 49 Saturation 8. 9. 10 volume. molar 61. 62 Solid state 1. 50. 56, 57 ~ a t e r , ~ t r i p point' le 30; vapor prer. Solubility 5. 8, 9, 10 sure 17: solubility curves in 8Solutions 17. 60 9, 10 Spectra 52. 54 Weight of gases 43 Steady state 34 Wood's metal 14 S t e m distillation 21 Stoiehiometry 2, 3 Zone refining 6
Precautions
Some general warnings are needed in handling the tubes. Safety glasses should be worn. Any sealed tube which is t o be heated (always do so gently, waving the tube back and forth through the flame) should be evacuated before sealing and should not generate noncondensable gas while sealed. Boiling occurs at lower temperatures under vacuum, so tube holders or gloves are needed. Evacuated tubes provide no cushion to the movement of condensed phases, so shaking such tubes is apt to break the tube when the accelerated phase of crystal or liquid hits the glass. Shake sidewise only. We wish to thank the NSF Science Course Improvement Program for a grant which supported a great many students and the author in the exploration of these experiments. Brief Directions for Some Sealed Tube Experiments Arranged by Classes Heterogeneous Equilibria Cn~stal-Crystal **I. (I)% HgIp is red at room temperature, hut turns yellow when heated and then red when cooled. There must he a change in crystal structure during heating. AH must he positive, AG must he zero at phase change, so A S is positive. For a crystalline phase change of positive AS, AV is probably positive (higher disorder) so (dP/dT) = AHITAV = positive (probably). AV can he observed to be posit,ive under a microscope for a single crystal, or by pressing on coaled yellow crystals and noting they burn red where pressed. Crystal-Gas 2. (V) C U ( N O ~ ) ~ . ~when I I ~ Oheated turns green, then black, producing obvious amounts of water and NO1 plus HNOa.
2
Number of psrentheses indicated tube design to he used fig.).
(see
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Mass is conserved (check by weighing), intermediate campounds form, and the system reverbs to the original salt if cooled for a few hours. 3. (V) CnS04.5H*0, forms intermediates, shows escape of gas by "dancing" of small crystals on surface and turns white (CuSOJ if liquid N1 surrounds other limb. Reactions reverse if tube stands at room temperature for a. few hours. "4. (I) (a) In,( b ) NHICI, ( c ) As. all sublime when heated. No a.ppreciahle wlor appears in evacuated tubes since rate of gas transfer is so high. Slow sublimation of In at room temperature leads to a single, large crystal due to nucleation limitation and tendency to minimize surface area. NH&I sublimes via NHaIg,and HCI(,, as ir spectrum shows. (d) An especially effective experiment is to place identical samples of IS in two tubes one of which is completely evacuated while the other contains 10-20 cm pressure of air. Heat the tubes simultaneously and equally. One forms a layer of purple gas at the bottom, and the iodine liquefies. The other remains almost colorless and the crystalline iodine "chatters" on the bottom loudly enough for the class to hear it. Condensation of iodine is noted higher up in this tube. Have class explain all the observations in terms of molecular behavior, i.e., molecular vaporization, collisions, and diffusion. Discuss AH, AS, AG. CrystabLipuid 5. (I) Fine crystals of NaCl in contact with saturated queous solution disappear and large cubic crystals grow as surface area minimizes; change is faster at elevated temperature. 6. ( I ) Zone refining of naphthalene, CloHs, plus b e n d , (CsH.CO)., or ezulene, ClaH8, proceeds visibly in s. Baird and Tatlock (Chadwell Heath, Essex, England) Multizone Refiner (-8350). 7. (111) Liquid Hg immersed in dry ice or liquid NP crystdlizes, then melts es temperature rises. Example of triple point,, thermometric fixed point. 8. (I) 10 g NaCl 25 ml HzO dissolves completely at about 80% illustratine limited soluhilitv and small rise in soluhilitv with rising temperature, so small, positive AH,,I., since hydration energy < lattice energy. 9. (I) 10 g KNOa and .55 g KNOs 25 ml Hz0 (in separate tubes) dissolve at about 50°C and 9 0 T , respectively, showing larger solubility (AG.,I.) and larger change with temperature than for NaCl (AH,,l, more positive). Larger AH,,l, is probably due to smaller AHhyd of larger NO8- then is true of the smaller C1- ions. Larger solubility probably due to larger AS,,I. for NOa- ions.
+
+
10. (1) 8 g Ca(0Ack in 25 ml H,O gives saturated solution which precipitates Ce(OAc)2when heated. AH.,I. is negative. '11. (I) Aqueous systems containing (a) NaOAcc, or (b) NsnSsOa.5HsO(., can be saturated by heating to 6&70°C, then supersaturated on cooling, and precipitated by shaking to start nucleation. May he nucleated by adding tiny crystal from outside if tube is open. Tube contents set up "solid" during precipitation. Note, by feeling tuhe, that AH is negative for precipitations; tie in with AS for the crystallihg system being negative, while AS for the surroundings and the universe are hoth positive. 12. (111) 3 g urea in 40 ml HnO immersed in salt-ice mix give. a freezing point. lowering from which the molar weight of urea can be calculated. 13. (111) For each of the following systerns nine or ten tuhes of varying concentrations give cooling curves (time versus temperature) from which the phase diagram (simple eutectics), the melting points of the pure materials, and the AH,., of each component can he cdculated: (a) Bi-Sn, (b) cinnamic acid-bensoic acid, (e) naphthalene-diphenyl smine. Class can use data to cadculate phase diagram, AH,.., ideality of solutions. 14. (I) Woad's metal (50% Bi, 27% Pb, f3% Sn, 10% Cd) melts readily in hot water (mp, 65"C), sol~d~fies almost as a quaternary minimum melting point system. Gas-Lipuid
"15. (Special) A commercisl dipping bird is s fine "problem" in analysis of B "black box," and is a heat engine everyone can understand. Water placed on felt-covered hesd evaporat,es, cooling head and lowering pressure of liquid inside. Higher pressure in lower end of bird (due to higher temperature there) forcesliquiduptowardheadoverbalancing bird whichdips forward wetting head in beaker of water. At same time lower end of large center tuhe comes out of lower level of liquid so pressure in two gas reservoirs equalizes and liquid runs down into lower bulb cooling and condensing some gas there. Process repeats as more H20 evaporates from head. Over a complete cycle the bird is unchanged, but heet has flowed from the bottom (the air is the hot reservoir) to the head (the felt is the cold reservoir) by way of the vaporization and condensrttion of the fluid
The steady state in the "cold" reservoir is maintained by the evaporation of HzO from the felt. Thus a more realistic measure of the overall efficiency would be the ratio of work done to the heat used in vaporizing the water. **I& (Special) Here at last is an experiment giving direct visual observations calculable from the Boltemann distribution equation. Perfluoropentane, CaHn, boils at about 3 5 T at 1 atm. (a) Sealed in an otherwise evacuated cane made of glass tubing, the CsFtzdistills from the handle of the cane to the lower tip due to gravity. About '/*" distills per day in a one foot cane. (b) A U-tube or ( c ) circlet, take longer (about 0.1 in./day), hut are even more convincing since there is no chance of a difference in temperature in the two limbs. The molecules "fall downhill" until the two Levels are the same. AP can he calculated from the Boltzmann relationship, and diffusion theory used to check the experimental results. Discuss AH, AS, AG. *17. (11) The following set of manometer tuhes allows the determination of the vapor pressure of a pure substance and its variation with temperature ("a" tubes), the lowering of the vapor pressure by nonvolatile solutes ("b" tubes), the heat of vaporization of the pure substance [either from dp/dT ("a" tubes) or from dp/dX ("h" tubes)], and the variation of vapor pressures and AH,., due to varying intermolecular forces. (a) HnO, CzHsOH, COHO,CsHlr in separate tubes whose temperatures are varied show the effects of H-bonding, r-r interactions and van der (b) Varying conWaals forces on vapor pressure and AH..,. centrations of CdHsCloHs, H20-NaCI similarly show the effect of a nonvolatile solute. (c) HnO-CbHsshows additivity of p's for mutually insoluble, volatile substances. (d) H20-CnH60H, C;HrC8Ht2 show approximate idealit~esand deviations therefrom. These tuhes are best used hy evacuating the system (just before or during class) with the stopcock in the Hg-manometer closed and the other two open. Close the one to the liquid reservoir as soon as the liquid hails and drivw out air. Close
the stopcock to the top of the manometer as soon as it is evacuated. Carefully open the stopcock in the manometer and let it come to equilibrium, then close this stopcock. This procedure minimizes any effectsdue to leaky stopcocks (since the one in the mercury manometer will not leak). Class can take data on p as a function of T and/or X and calculate AH,,, ~. Tho. ., and P versus T curves. Discuss AH. AS, AG. 18. (111) MP (triple point) of sulfur shown in sealed, evacuated tuhe; mp and bp if tuhe is left open. 19. (IV) Drop of volatile liquid [(a)8 0 , (b) CsHe] evaporates, condenses, shows dew farnabion and cloud chamber effect as piston is expanded and contracted. Gas or liquid completely disappears if V changes greatly, hut P is independent of V if changes are small and slow. 20. (V) Cooling "empty" arm of cryophorus when other arm contains liquid HsO causes (1) initial boiling as P drops, (2) top layer of water to freeze as A H , comes from cooling of surface, (3) small ice particles to "dance" on frozen layer as ice sublimes. Examination of cold end shows ice. 21. (VI) (a) 5650 HSO-CaHasystem shaken in warmed end of cryophorus tube (other end cooled in liquid Nn) leads to disappearance (vaporization) of hoth HIO and CeHs, but CaHe disappears completely long before H20 is gone. The ratio of the two vapor pressures can he estimated by relative rates of e v a p oration. The ssme effect is noted if one end is heated and the other left at room temperature, illustrating steam distillation. (6) Hz0 and a-nitrophenol also steam distill readily away from p-nitrophenol. **22. (VI) Sealed tuhes of COI: (a) 'Ao full of liquid, ( 6 ) full of liquid, ( c ) *Aofull of liquid behave differently on heat,ing. Liquid diminishes in volume and disappears in (a), gas phase diminishes in volume and disao~esrsin ie). , ,, volumes of both phsses remain about the same in (b) and meniscus disappears at Tc. Do not drop tuhes and do not overheat any, especially (c) since it is in the supercritical region where P rises rapidly as T increases and tubes may explode. Note indistinguishability of gas and liquid phases unless hoth are present. Contrast distinguishability of crystals on basis of repeating structure. Discuss fact that all properties of liquids and gases become Tc. identical at Tc; A (all funet,ions) 0 as T '23. (I) Liquid Hg covered by ground blue glass (about 1-2 mm mesh) is an old favorite, and should he. Heat the top of the tube to minimize later condensation then heat bottom and note jostling of beads as Hg begins to evaporate, then "levitation" of glass heads 2-5 em (or more) above liquid as boiling occurs.
..
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Lip&-Lipuid **24. (I or 11) (a) p-hoxyanisole and (b) stiihestrol give liquid crystals. (b) gives beautiful interference patterns. (a) gives muddy brown liquid crystals on melting (muddiness indicates presence of large scattering centers for light), then clear dark brown liquid an further heating (clearness of liquid indicates small aggregates). Both transitions are at fixed temperature (1 component, 3 phases, F = 0). Both liquid crystals owe their formation to molecules with one dimension longer than either of the others. This leads to lining up (crystsllinity) in the low temperature liquid phase. Discuss AH, AS, AG. 25. (I) (a) 15 g Phenol, 10 ml H>Ogo from two to one phase at about 55'; (b) 10 g phenol, 15 ml H10 at about 72'C. Discuss AH, AS, AG in terms of intermolecular bonding snd phase change. (c) 5 g (C2Hs)sN-20 ml HzO, ( d ) C.H,,OH-glycerine, and ( e ) CHICOON-CS1 behave similarly. Note that in every case a substance having strong H-bonds with itself (H1O, glycerine, CHsCOOH)finally, s t higher temperature, farms a.solutlon with a substance less polar than itself. **26. (I) 10 g nicotine15 ml HsO (one phase at room temperature) form two phases when heated. The strong intersubstance H-bonds tending to give compound formstion at lower temperatures, break at higher ones. AH is positive (bonds break), but AS is also positive since molecules are freer as temperature rises. Note this change is consistent with the general tendency of molecular aggregates to become simpler at higher temperatures. An especially effective experiment is to start with the (C*H,),NHnO tuhe from Experiment 25 and this tube at room temper* turn. Ask class to wediet effect of heating. Heat tubes. Tube from 25 forms one phase at about sametemperature as tube here forms two phases. This gives a nice puzzle, especially Volume 47, Number
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as each change occurs at a fixed T (C = 2, P = 3, pressure fixed, so F = 0). Discuss AH, AS, AG. 27. (I) (a) 10 g (CnHshN, 5 g (CH8)&O, 10 ml HIO gives two layers (3 components) with consalute T 40DC. (b) 10 g C6HbOH,5 g (CHa).CO, 10 ml H.0 does same at T T Sac. Note similarity and differences in molecular types and intermolecular bonding tendencies within and between the two sets. 28. (I) (a) 20 ml Anhydrous CIHbOH and 40 ml bicyclohexyl have a critical solution T = 23.4'C; (b) with 1% H.0, T = 41.4'; (e) with 2% H.0, T = 54.1DC. This system can quickly and accurately determine the purity of nearly pure alcohol contaminated with H20.
Three-Phase Systema Na2S0, and ssturated aqueous 29. (111) NsnSO,.lO H 2 0 solution at 32.38"C. Thermometric fixed point; C = 2, P = 3, fixed pressure, F = 0. *30. (111) Triple point of water in evacuated tuhe containing crystalline, liquid, and gaseous HzO; C = 1, P = 3, F = 0. Rotes of Reoction
31. (VIII) (a) Salol, ( b ) thymol, ( c ) supersaturated Na&Os. 5HI0 all undercool readily. When nucleated, they have rates of crystalliaatition which depend on ambient temperatures. The rates inereme at first as T falls below the respective melting points (since AH,., is removed more rapidly), at lower temperatures the rate becomes almost independent of T (since rate determining step is now conduction of beat through the melt), and at still lower temperatures the rates begin to decrease (due largely to viscosity effects and diffusion becoming rate-determining). 32. (Special) An old tungsten lamp bulb (preferably clear glass) shows a tungsten deposit at the region which was ahove the hot filament during the burning life of the lamp. The tongsten was carried there by evaporation into the surrounding NI or Ar which returned most of the metal to the filament but carried the rest,,by convection, to the observed spot. This is one reason tunesten lamus are not evacuated and that they are supposed
*34. (Special) A sealed goldfish bowl containing a fish, a snail, and some aquatic, oxygen-producing plants, will remain in a steady state for about a month. Consult your local tropical fish expert for appropriate species. *35. (IV) Rapid compression of NOn in a piston leads to intensification of color (due to adiabatic rise in T ) rather than the expected fading of color, which does follow shortly as T falls. N204 equilibrium (This ~-~ effect is not due to a slow 2NOn which is known to he very rapid from other data.) ~~
~
~
Dynomic Equilibrium
=
**36. (I) Tubes of 2NOn N20, (a) in ice, (b) at room temperature, (e) and in hot water m e another long-time, justlyrespected favorite. Discuss free redicals, and AH, AS, AG in terms of molecular hehsvior and bonding. See also Experiment 35. Discuss color in free radicals and "normd'l" molecules. 37. ( I ) Hexephenylethane in benzene deepens in color when heated, reversing when cooled. Discuss free radicals, and AH, AS. and AG in terms of bonding and molecular behavior. h"
'38. (I) Fes+l.ql
+ thi0nine.S.
Fe3+l,1
aarK
+ reduced thionine
can be studied with 50 ml HsO, 1 ml 0.001 F thionine, 1 ml 6 F H2SO,, 0.2 g FeSO, or Fe(NH,)n(SO,)z. The resulting lavender solution can be bleached in s few seconds by a. photo flood, but will recover its color in several minutes in the absence of strong illumination. Bet,ter to use stopcock closure than sealed t,ube,'and pump out air. 39. (Special) H.lgl Inlll 2HIlgl. We have not built this one yet but hope others will try it. The main tuhe contains HX.I whose pressure can be measured by the H2which diffuses through the Pd plug into a Hg manometer side tube. pr?is regulated by adjusting T in another sidearm containing Im). T, is higher and adjusted by heaters in main tube. Patdetermined by glass (click or sickle) gauge on main tube. Use data to determine K,,as a function if% 40. (I) (a) Aqueous 1 M NaOAe plus phenolphthalein turns colorless on heating due to fall in pH as Khyd changes. (b) Brom-
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thymol blue in very pure .H1Oalso changes color as K, varies with T. Properties of Fluids
*41. (Speeial) A plain R-mm tube about 80 em long with stopcock at the top and its lower end in a s m d reservoir of mercury makes a fine barometer. Evacuate the tube, mercury rises. Close stopcock. Measure h. Tilt tuhe gently (sa Hg moves slowly), hear "click" 8s Hg hit? end of tube. Heat center of Hg to show change in h but not in P. 42. (Special) Place board on l-gal. can. Stand on board to show strength of can. Add 10 ml H20, heat to fill can with steam, seal with screw top, cool under faucet. Account for changes. 43. (Special) Evacuate a 1-1 bulb with a stopcock closure. Weigh it. Let in s gas (air, natural gas, other). Weigh again. Heat with stopcock open. Close stopcock. Cool. Weigh again. 44. (Special) Gas thermometers e m be made from: old thermometer tubes in which a 2-3 mm thread of Hg is left; copper flush tank bulbs connected to a &20 lhs/inQauge; or flasks connected to a mercury manometer. Either constant pressure or constant volume types are useful. '45. (Special) A porous clay cup connected by a rubber tube to a. manometer filled with colored fluid, or to a beaker of colored solution allows experimentation on relative rat,es of diffusion of air, Hz, COT,and other gases. Surround the clay cup with a beaker containing the desired gas and note change in pressure and rate of change of pressure. Remove beaker, note reversal of effect. '46. (Special) 6 M NH3 and 9 M HC1 placed at opposite ends of a 1.5-mm glass tube give sn initid precipitate consistent with Graham's law in about 20 min. Allow precipitation to continue to obtain Liesegang rings due to rhythmic precipitation from the gas phase. Concentrations of aqueous solutions must be such as to give same partial pressures of NHa and HCI. 47. (Special) A radiometer's rate of rotation is proportional to edge length of the vanes, not to their area, consistent with a mechanism involving thermal transpiration of molecules from the cold to the hot side of the vanes. [J. CHEM.EDUC.,38, 480 (1961).] $48. (I) Viscosity effects in sulfur create less smell if done in a. sealed tuhe. An open tube allows simultaneous observation of effects of chilling liquid to produce rubbery sulfur. 49. (I) (a) Ether, (b) C6He,( c ) HzO, ( d ) 18 F H%SOd,( e ) glycerine, swirl and flow so that their relative viscosities can he observed and correlated with intermolecular bonding. Electric01 and Electronic Effects
50. (Special) Obtain a. clear, tungsten filament light bulh, immerse it in a molten bath of NaNOa[l, in a steel crucible which is the anode while the hot tungsten filament is the cathode. Ten to fifteen volts should be adequate. Sodium ions will migrate through the glass and be reduced to metdlic sodium which will collect as a mirror on the inside of the bulh. 51. (VI) Slowly evacuate tube as Tesla, coil energizes electrodes. Note changes in electrical discharges within tube. *52. (Special) Excite Geissler tubes of several gaseous elements. Observe over-all colors. Observe line spectra with small plastic replica gratings. "53. (Special) Experiment with charge, momentum, and linear flow of electrons in gases using a Tesla coil to excite Crookes' tubes. [J. CHEM.EDUC.,38,480 (1961).1 54. (I) Molten K I emits red light at s. temperature far below that at which other substances are "red-hot." 55. (I) .(a) Sparking NaCl crystals with a Tesla coil gives discoloratmn which disappears on heating (with light emission as electronic imperfections form and disappear in crystal). (b) Glass from old X-ray tubes or from hot,tles which have been exposed to lenethv radiation (as from UV in the mountains) also emit .. ...~-.~-, light when warmed. 56. (VI) (a) AgI conducts due to mobile Ag+ starting at 155°C (far below the mp of .5.5fiaC). Onset of conductivity is coincident with change in crystal stmcture (147'C). The high temperature, conduct,ing phase consists of a. body-centered Izttice of I- with the cations randomly distributed interstitially. decreases 12% when I t is interesting to note that the condnctivit,~ the crystals melt. (b) Ag>HgL also changes crystal structure (,50°C) and shows cationic conductivity when heated. Color also changes ~~~~
~
57. (IV specialized) The conductivit,y of Co wire is a function of the partial pressure of the surrounding O1 as the composition of the oxide layer varies. 58. (Special) Sequentially place between the poles of a strong permanent magnet sealed vials of (a) MnS04ie,, (b) FeSOm, ( e ) CoS04i.l, (d) ZnS04i.l hung on a long string. Slowly pull magnet away and measure displacement of vial before i t is pulled out of the field. This crude Guoy balance allows est,imate of numbers of unpaired elect,rans. [5200 gauss magnet from Herback and Radamon, 1204 A x h St., Philadelphia, Pa. 19107.1 Optiml Effeds
59. (I with flat ends) Measure optical rotation of solutions placed between Polaroid discs. Note variation with concentrat,ion and with substance. 60. (I) (a) Dissolve 2 g CuSO. in I liter of water and add concentrated aqueous NHs until precipitate just barely dissolves. ( b ) Shake 3 g of finely ground ultramarine in 1 1 IIsO and adjust two concentrations until colors mabch. Fill tubes and nse Tyndall effect and settling to discriminate solution and suspension. Use to differentiate "clear" from "colorless."
Exhibits
61. (I)One male samples of all available elements. 62. (I) One mole (or one equivalent weight) samples of (a) CuO, ( b ) CmO, (c) HzO, (d) H Z O (simulated ~ or unsealed), (el CB, (f) CClc, ( 8 ) CaCIz, ( h ) CaO, (i) CaH*, (j) PbO, (k) Pb,O,, ( 1 ) PbO*. 63. (I) Samples of the final precipitates, or other storable tests, for the qualitative analysis scheme. [e.g., all the insoluble chlorides and sulfides.] 64. (I) Samples of typical compounds (nibrates, chlorides, and oxides m e goad) of a%many elements as possible preferably arranged in a. periodic table display to show trends in color, phase, general appearance. Samples of ares, common proprietary chemicals, 1 F solubions of all reagents which might he used in classroom experimenbs. Every solution label should contain the name, formula (including waters of crystallization), formula weight, and any special directions required in making up the solution.
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