Demonstrations for teachinglaboratory safety

By E.G. Senkbeil and P. Crisp

INTRODUCTIONLaboratory safety should be taught asan integral component of an under-graduate chemistry curriculum. Wehave found that students need morethan just a list of ‘‘Do’s and Don’t’sof Laboratory Safety’’. Students needto understand the chemistry principlesbehind safety concepts and be able toapply them in a logical and responsiblemanner. We have used a series of che-mical demonstrations in the classroomthat not only stimulate the student’sattention but also help to better explainsafety concepts. This article describesthe chemical demonstrations and theirrelevance to understanding safety prin-ciples.

All demonstrations should be prac-ticed in advance several times to helpinsure good results and safe workingconditions. References should bechecked for complete details in pre-paration, setup, and safety concernsfor the demonstrations described. Allappropriate personal protective equip-ment should be worn, including che-mical splash goggles, laboratory coator apron, and gloves at all times.Demonstrations should be performedbehind an appropriate sized safetyshield. Any demonstration involvingflames should be done in a well-ventedroom (or hood when applicable) and afire extinguisher should be availablenearby. Material Safety Data Sheetsshould be reviewed for each chemical

used, especially noting the safetyissues. ‘‘Safety Tips’’, described foreach demonstration, are intended toprovide helpful safety informationabout the demonstration, but not allpossible safety issues can be antici-pated. Although they are useful teach-ing tools, no demonstration should bepresented if it poses a serious hazard toindividuals, either because of the lackof the appropriate facilities or equip-ment or inherent safety issues.

Plastic Coated Bottle DropSafety Tips/Cleanup—Sweep the floorto remove any traces of broken glassfrom the shattered bottle, especially ifthe cap cracks and is released. If theshattered bottle is passed around forstudent inspection, place it in a con-tainer so that students do not handleany sharp glass edges. The shatteredbottle can be disposed in an appropri-ate broken glass container.

Materials—Chemical splash goggles,laboratory coat or apron, gloves, empty500 mL plastic coated glass storagebottle.

Preparation—None.Demonstration—Drop the empty

plastic coated bottle (with lid) from aheight of approximately 5 feet behind asafety shield positioned on the floor.

Discussion—Plastic coated bottlesare recommended for storage of hazar-dous chemicals such as toxics, acids,and bases. If dropped or bumpedagainst a bench, the glass might shat-ter, but the plastic protective coatingwill protect against a major spill andcontain the hazardous liquid and theshattered glass. It works best to do thisdemonstration unannounced to stu-dents, or as a ‘‘fake’’’ accident. Theresults may be a shattered bottle orsometimes an unbroken bottle, bothbeing an example of containmentdue to the plastic coating. Occasionallythe bottle doesn’t break, but the capcracks and comes off of the bottle. Thisdoes suggest that the plastic coatedbottles are not ‘‘fool-proof’’ in prevent-

ing spills. For this reason, the demon-stration is performed without liquid inthe bottle.

Burning Book1

Safety Tips/Cleanup—Protective eye-wear should be worn. Be careful tohold the book away from you and atabout a 458 angle. This will keep you adistance from the fire, and will notallow the students to see the ignitionbox. Use only lighter fluid; other moreflammable liquids may cause a moreuncontrollable flame or severe explo-sion. Always have a fire extinguisher inclose proximity.

Materials—Burning book (see refer-ence 1 for construction), lighter fluid,chemical splash goggles, laboratorycoat or apron.

Preparation—Glue a paragraphdescribing the combustion processinto the book below the ignition boxas shown in Figure 1. Place a smallamount of lighter fluid (3–5 mL) onthe wick in the ignition box of theburning book and then close. Thismay be done 5–30 minutes before theactual demonstration.

Demonstration—Ask students tocomment on the properties of combus-tion reactions. Tell students that youwould like to read a good descriptionof combustion reactions from a text(the Burning Book). Open the textand start reading. While reading, touchthe igniter and continue to read whilethe book is ‘‘burning’’. After comple-tion of reading, ask students how theyshould extinguish the fire. Per studentcomments, close the book and con-tinue the class discussion.

Discussion—In depth discussion ofthe components required for a com-bustion reaction can follow thedemonstration. What component isbeing removed by closing the book?The demonstration is useful in describ-ing combustion reactions and also tointroduce the idea of how fire extin-guishers work. Students are interestedin seeing the interior of the book which

FEATURE

1074-9098/$30.00 � Division of Chemical Health and Safety of the American Chemical Society 17doi:10.1016/j.chs.2004.04.001 Elsevier Inc. All rights reserved.

E.G. Senkbeil is affiliated withHenson School of Science andTechnology, Salisbury University,1101 Camden Ave., Salisbury, MD21801 (Tel.: (410) 543-6455; fax: (410)543-6083; e-mail:egsenkbeil@salisbury.edu).P. Crisp is affiliated with School ofChemical Engineering and IndustrialChemistry, University of New SouthWales, Sydney, NSW 2052, Australia.

can also lead to discussions of volatilityand insulation.

Sugar Dehydration with SulfuricAcid2

Safety Tips/Cleanup—The reactionshould be done behind a safety shieldand in a laboratory chemical hood.Throughout the experiment, personalprotective equipment (chemical splashgoggles, gloves, and apron) should beworn. The reaction proceeds moresmoothly with the formation of a con-tinuous carbonaceous column whenperformed in a beaker, rather than atest tube. The smaller radius test tubecauses the formation of gas pocketsthat break the column. After washing,the black column should be placed in abeaker for visual display and nothandled with the hands. Any excessor spills of sulfuric acid should be neu-tralized with an appropriate weak base(sodium bicarbonate) and then prop-erly disposed.

Materials—Granulated sugar, con-centrated sulfuric acid, two 50 mL bea-kers, safety shield, chemical splashgoggles, laboratory coat or apron,gloves.

Preparation—See instructions forpreparation from reference 2.

Demonstration—Add 20 g granu-lated sugar to the 50 mL beaker behinda safety shield and under the hood.

Add 15 mL sulfuric acid and stirbriefly. Note results. It may be neces-sary to support the growing black col-umn with a stirring rod. After thereaction is complete and cooled, washthe carbonaceous black column gentlywith water in a chemical hood (toremove any acrid vapors emitted).Allow the column to dry. Place thecolumn back into the beaker for stu-

dent inspection. The wash watershould be neutralized by the additionof weak base and discarded. Sincethis demonstration produces copiousamounts of acrid smoke, we recom-mend that the black column be savedand used for future classes, rather thanrepeating the production of the col-umn. We have used columns that are10 years old (see Figure 2), without anyproblems. It can be shown in compar-ison to a beaker containing the originalgranulated sugar.

Discussion—The significance of thisdemonstration is the visual results ofthe reaction of a strong dehydratingagent, sulfuric acid, with sucrose (oranother oxygenated hydrocarbon).The reaction of sucrose and sulfuric is:

C12H22O11ðsÞ þ 11H2SO4ðlÞ! 12CðsÞ þ 11H2SO4 � H2OðgÞþ heat:2

The heat released is due to both thedehydration of the sucrose and thedilution of the sulfuric acid. The blackcolumn of carbon occurs because ofthe escape of the water vapor throughthe black residue product.

Students utilize sulfuric acid in theirundergraduate experiences and needto be made aware of its hazards. Theyhave likely seen it ‘‘eat’’ holes into

Figure 1. Ignition box in the ‘‘Burning Book’’.

Figure 2. Carbonaceous black column produced from sugar dehydration withsulfuric acid.

18 Chemical Health & Safety, July/August 2004

paper towels or possibly clothing andmost students attribute this as a resultof sulfuric acid being a strong acid.This demonstration or visual displayclearly points out the effects of thedehydration process and accompany-ing release of heat. Since our skin iscomposed of a variety of oxygenatedhydrocarbons (proteins, sugars, fats,etc.), the sulfuric acid reacts with ourskin in a similar destructive manner.The demonstration and explanation ofthe reaction indicates why sulfuric acidis so hazardous when it comes in con-tact with skin.

Nonburning Dollar Bill3

Safety Tips/Cleanup—Wear chemicalsplash goggles, a laboratory coat orapron, and gloves when conductingthe demonstration. Be careful not toignite the paper above any beaker con-taining the flammable alcohol. Removeany bottles of ethanol (or other com-bustibles from the demonstration area).Ethanol is flammable; use with appro-priate safety precautions.

Materials—Chemical splash goggles,laboratory coat or apron, gloves, etha-nol (200 mL of 95% or 100%), safetyshield, twenty dollar bill, two 1,000 mLbeakers, two pairs of crucible tongs,two types of paper to combust, watchglasses (large and small), small pan,multipurpose igniter.

Preparation—Place 200 mL of etha-nol in one beaker and 200 mL of waterin the other beaker. Label each beaker.Cover each beaker with plastic wrapand secure the wrap with rubberbands. Be sure to have a dollar billavailable in case the audience doesnot supply one.

Demonstration—Pick up a piece ofone type of paper and dip it into thebeaker containing water. Place over anopen pan and try to ignite. Note to theclass that of course the wetted paperwon’t burn. Pick up a piece of the othertype of paper and dip it into the beakercontaining ethanol. Place over an openpan (be sure not to ignite over theethanol beaker since the burning alco-hol will drip back into the beaker andignite it) and try to ignite. Act surprisedto see that the paper burns. Extinguishthe paper by covering with a watchglass and suggest to the class that theproblem why it burned when wetted

was because of the type of paper used.Ask the class to provide you with atwenty dollar bill to retry the experi-ment. While coaxing the audience toprovide a (twenty) dollar bill, pour thewater and alcohol together withoutany comments and mix by shaking.Dip the bill (either yours or one thathas been provided by the audience)into the mixture, hold over the pan,and ignite. Note what happens and askthe class to explain the results.

Discussion—The demonstration isgood to test the class on what theyobserved during the experiment andwhy the paper bill did not burn. Paperthat has been wetted with water willnot burn. Paper that is wetted withethanol will burn (both paper andalcohol are combustible). When paper(dollar bill) is wet with 50:50 ethanol/water mixture, it will not burn. Thealcohol will burn off of the paper butthere is sufficient water remaining sothat the paper will not burn. The waterin the mixture keeps the paper belowits ignition temperature. The waterabsorbs some of the energy of combus-tion of the ethanol as it (the water)heats to its boiling point and is vapor-ized. The paper will still be wet withwater when the alcohol has burnedaway. This can also be used as anintroduction to the use of water as afire extinguishing agent and why it is

still used to protect wooden structuresaround flammable liquids fires.

Dust Explosion Demonstration4,5

Safety Tips/Cleanup—The dust explo-sion apparatus should be placedbehind a safety shield in a well-ventedroom or laboratory chemical hood. Analternative source for the air pressurethat is required to create the dustwithin the coffee can is a small airpump. Lycopodium dust is very explo-sive and works well in this demonstra-tion, but inhalation and skin contactshould be avoided due to possibleallergic reactions and other unknowneffects.5 It is not recommended for use,unless done outside in an open area orunder a hood.

Materials—Chemical splash goggles,laboratory coat or apron, gloves, driedflour or cornstarch, lycopodium pow-der (optional), 3 lb. coffee can withplastic lid, small votive candle, largerubber stopper, rubber tubing (1.5 m),small plastic vial, small cardboard fro-zen juice can, watch glass, multipur-pose igniter.

Preparation— See reference 4 forconstruction of the dust explosionapparatus. See Figure 3 for a view ofthe interior of the dust explosion appa-ratus/coffee can.

Demonstration—Place a smallamount of dried flour or cornstarch

Figure 3. Side view of dust explosion apparatus.

Chemical Health & Safety, July/August 2004 19

on a watch glass and try to ignite it witha multipurpose igniter. Note theresults. Place the dust explosion appa-ratus behind a safety shield. Place 3–5 g of dried flour or cornstarch in thevial inside the juice can within thecoffee can. Ignite the candle which isplaced on the stopper on the oppositeside of the interior of the can. Place theplastic lid (with small hole in it) on thecan. From a distance, blow air throughthe rubber tubing to disperse the pow-der within the can. Note the results.Lycopodium powder may be used inplace of the flour or cornstarch and itworks very well but safety concernsshould be noted as described earlier.

Discussion—Factors that affect thereaction rate is an important safetytopic. The flour or cornstarch willnot effectively burn if placed on awatch glass due to limited surface areacontact between fuel and oxidizer. Thedust created in the apparatus greatlyincreases the surface area of air withthe small combustible dust particles,creating the explosion which will blowthe plastic lid off of the can. This can bedirectly related to dust explosions ingrain elevators that are often noted inthe news. The powder must be driedfor an explosion to occur. The candleand plastic lid must be replaced peri-odically due to damage from the explo-sions.

Combustion of Cellulose vs.Nitrocellulose6

Safety Tips/Cleanup—The preparationof the nitrocellulose must be doneunder the hood with the appropriateprotective equipment (chemical splashgoggles, laboratory apron or coat, andrubber gloves). Care should be takenwhen handling the acids. Properly dis-pose of the chemical waste. Safety eye-wear should be worn when igniting thecotton balls or nitrocellulose. Thedemonstration should be performedin a laboratory chemical hood due tothe heat and smoke produced. Thenitrocellulose can be made 3–5 daysin advance and stored in a spark freeand dry environment. Unused nitrocel-lulose (stored up to one week) shouldbe destroyed by ignition of small piecesin a hood.

Materials—Chemical splash goggles,laboratory coat or apron, gloves, cotton

balls (cellulose), concentrated H2SO4,concentrated HNO3, 1,000 mL beaker,ice bath, rubber gloves, multipurposeigniter, metal tray, sodium bicarbonate.

Preparation—See reference 6 for pre-paration of nitrocellulose (cellulose tri-nitrate) from cotton balls.

Demonstration—Place two smallcotton balls (cellulose) on a metal tray.For comparison of combustion rates,pack one cotton ball tightly into asmall ball and puff up the other to alarger volume. Hold the cotton ballsseparately with tongs and ignite overthe metal tray. Ask students to noteand then explain the results.

Now place two small balls of nitro-cellulose (prepared from the cottonballs as referenced earlier) on the metaltray. Ask students to note any differ-ences in the appearance of the nitro-cellulose as compared to the originalcotton balls. Repeat the procedure inthe previous paragraph to compare thecombustion rates of a packed nitrocel-lulose ball and a puffed nitrocelluloseball. Ask students to note and thenexplain the results.

Discussion—The comparison be-tween the combustion of the com-pressed cotton ball and the puffedcotton ball clearly demonstrates theeffect on reaction rate by increasingthe surface area between the fuel andthe oxidizer. This is an important fac-tor in considering the relative reactiv-ity and safety concerns of chemicals invarious physical forms.

Students note that the nitrocellulosehas no different physical appearancefrom the cotton balls, but burns at amuch more vigorous rate. The signifi-cance of having the oxidizer and fuelsource within the same structure canthen be discussed while noting theactual chemical structure of nitrocel-lulose. If the nitrocellulose is fresh, therate of combustion between the com-pressed and puffed balls should be thesame. Students are asked to explainwhy. This demonstration of combus-tion of nitrocellulose can lead into adiscussion of explosives and character-istics of explosive reactions, an appro-priate discussion for laboratory safety.It is interesting to note to students thatthe heat of combustion of trinitroto-luene (TNT, 15 MJ/kg) is much lowerthan the heat of combustion of pro-

pane (50 MJ/kg).7 There is much lesstotal energy available from the TNTbut the rate of the combustion reactionis enhanced due to the presence ofboth the fuel and oxidizer within thesame chemical structure. This createsthe explosion or blast potential due tothe rate enhancement and its effects onthe rapid expansion of the gas productsin the exothermic reaction.

Flame Tornado8

Safety Tips/Cleanup—The demonstra-tion should be done in a laboratorychemical hood. The phonographshould be on a flat surface and the wiremesh securely positioned on the turn-table by use of the tongue depressors. Awobbly rotating screen cylinder on theturntable is a fire hazard. Be sure tohave the evaporating dish readily avail-able when needed to smother the firein the mortar.

Materials—Chemical splash goggles,laboratory coat or apron, gloves, nitro-cellulose, ring stand and ring, lighterfluid, cloth rag (10 cm � 10 cm), largetongue depressors, small mortar, largeevaporating dish, Velcro strips, porta-ble phonograph, multipurpose igniter,aluminum screening.

Preparation—Prepare the nitrocellu-lose as described in reference 6. Thedemonstration setup (flame tornadoapparatus) is described in reference 8and shown in Figure 4. The aluminumscreening may be held in cylindricalform with short pieces of copper wire.The tongue depressors (or flat 10 cmsticks) are hot glued to the turntable tostabilize the aluminum screen cylinder.The Velcro is used to hold the mortarin place in the center of the turntable.

Demonstration—A ring stand is setup and nitrocellulose is spread on asmall ring approximately 1.3 metersfrom the base. Place the mortar con-taining the lighter fluid soaked rag on ablock (of equivalent height to turnta-ble) above the base of ring stand. Ignitethe rag and note that the flame will notignite the nitrocellulose. Extinguishthe flame with an inverted large eva-porating dish. Ask students to suggestways in which the nitrocellulose can beignited without raising the mortar orlowering the nitrocellulose. Addingmore fuel is a common answer butcan be shown to be incorrect. Now,

20 Chemical Health & Safety, July/August 2004

place the phonograph (previously con-cealed) on the ring stand base andattach the mortar (with lighter fluidsoaked rag) to the turntable with theVelcro. Ignite the rag, place the alumi-num screen wire cylinder (approxi-mately 1 meter in height anddiameter of turntable) within the ton-gue depressors glued to the turntable.Turn the phonograph on at low speedand then higher speed. Ask students tonote the results of the demonstration.The flame in the mortar is again extin-guished using a large inverted evapor-ating dish.

Discussion—It is noted that as thespeed of the rotation of the turntable isincreased the vortex of fire becomeshigher and higher. This demonstration

is a valuable tool in describing flameproduction. A flame is composed ofhot gases and therefore extendsupward, creating convection currentsaround it. The cooler, denser air abovethe flame sinks downward due to grav-ity. This will limit flame height. Whenthe flame is placed in a spinning frameof reference, the centrifugal force willhave a greater effect on the coolerdenser air and push it out away fromthe flame. The outward movement ofair toward the sides of the rotatingscreen creates a drop in pressure abovethe flame and allows the hot gases ofthe flame to move upward more easily,creating the heightened vortex of fire.9

The heightened flame should causethe ignition of the nitrocellulose but a

question to consider is if the flameactually has to touch the nitrocelluloseto cause its combustion. In most casesstudents note that the flame did nottouch the nitrocellulose before its igni-tion occurred. In fact, the heat from thenearby flame initiated the nitrocellu-lose combustion and can be used todiscuss the auto-ignition temperatureof flammables and combustibles.

Fire Starter Using SodiumPeroxide10

Safety Tips/Cleanup—The demonstra-tion should be performed behind asafety shield in a well-vented roomor in a hood. Any excess sodium per-oxide should be added to a smallvolume of water and the product baseappropriately neutralized with a weakacid.

Materials—Chemical splash goggles,laboratory coat or apron, gloves, anhy-drous sodium peroxide, cotton balls orgauze, dropper bottle of water, mortarand pestle, sand, safety shield.

Preparation—A sample (0.2–0.5 g) ofanhydrous sodium peroxide is groundto a powder and temporarily storedinto a dry vial, capped with a plasticstopper.

Demonstration—Place a fluffed cot-ton ball in a mortar that has been half-filled with sand. Place the mortarbehind a safety shield and sprinklethe sodium peroxide powder on thetop of the fluffed cotton. Slowly dropwater (4–6 drops) onto the cotton inthe mortar. Note that the results maybe delayed (15–30 seconds), depend-ing on the freshness of the sodiumperoxide.

Discussion—Discussion of oxidizersand their properties is a significanttopic in laboratory safety. The reactionof sodium peroxide with water:

2Na2O2ðsÞ þ 2H2OðlÞ! 4NaOHðaqÞ þ O2ðgÞ þ heat

demonstrates the ability of an oxidizer(Na2O2) to act as an oxygen source. Inthe presence of a fuel (cotton) andsufficient heat from the reaction, com-bustion can occur. Students are inter-ested in the fact that the addition of asmall volume of water actually initiatesthe combustion process. This reactiondirectly relates to an accident reported

Figure 4. Flame tornado apparatus.

Chemical Health & Safety, July/August 2004 21

in a high school where the studentswere reacting sodium peroxide withwater to measure the molar volumeof oxygen gas in a laboratory experi-ment. Upon completion of the lab,students wiped their desks with damppaper towels. Later, some wastesodium peroxide and damp papertowels ignited the contents of a trashcan. No serious accident occurred,other than a charred floor and ceilingwhere the trash can stood.

Cryogen Demonstrations11

Safety Tips/Cleanup—Chemical splashgoggles, laboratory coat or apron, andimpervious protective gloves should beused at all times when handling cryo-gens. Cryogens should be transportedin standard Dewar flasks with loosefitting lids. Never use common Ther-mos jugs or other glassware. Demon-strations with liquid nitrogen shouldbe done in a well-ventilated area. Inthe case of a liquid nitrogen spill, therapid evaporation of the liquid nitro-gen and expansion of the nitrogen gascould cause asphyxiating conditions.The test tube or cup for collectingliquid oxygen should be thoroughlycleaned with water, avoiding any con-tact or presence with combustibleorganics. Due to its high degree ofreactivity with combustibles, do notcollect any more liquid oxygen thannecessary for the experiment. Anyexcess should be placed under thehood and allowed to evaporate. Ifliquid oxygen or oxygen-contaminatedliquid nitrogen is spilled on a combus-tible solid (such as clothing, wood, orsome other hydrocarbon), a violentexplosion can occur in the presenceof an ignition source.12 Extreme pre-cautions should be used in the produc-tion of even small quantities of liquidoxygen.

Materials—Chemical splash goggles,laboratory coat or apron, protectivematted gloves, liquid nitrogen, clean1 gallon metal paint can, two Dewarflasks (large and small), clean largediameter stainless steel test tube orcup for collecting liquid oxygen, safetyshield, two watch glasses, small floweror other article for freezing.

Preparation—Set up a liquid oxygencollection system as shown in Figure 5.The metal gallon container must be

secured to the ring stand, and at anangle so that any condensed liquidsfrom the exterior drips into the testtube or cup cooled in liquid nitrogen.

Demonstration—(a). Place a floweror some other appropriate material in aDewar flask containing liquid nitro-gen. After about 30 seconds, removethe flower with tongs, and drop it intoa clear glass container. Ask students tonote results.

(b). Place approximately 2 L ofliquid nitrogen into the secured metalpaint can.

Make sure the collecting stainlesssteel test tube or cup has been cooledin liquid nitrogen contained in a smallDewar flask. Ask students to carefullynote what happens. Collect approxi-mately 5 mL of the liquid droplets inthe test tube or cup (5–10 minutesrequired). Slowly pour the collectedliquid onto a small piece of puffedcotton ball placed on a watch glassbehind a safety shield. After a few sec-onds, to allow vaporization of anyliquid on the watch glass, ignite thepuffed cotton. Ask students to explainthe results.

Discussion—Part ‘‘a’’ of the demon-stration is used to describe the rapidfreezing that occurs with the use of acryogen, such as liquid nitrogen. Thiscan be related to the discussion of theuse of cryogens in the laboratory andtheir common safety hazards, such asthe rapid freezing of tissue.13 Propersafe handling of cryogens should bediscussed at this time.

Part ‘‘b’’ can initially be used as an‘‘inquiry-based study’’ to explain whatis occurring on the exterior of the canfilled with liquid nitrogen. Dry air iscomposed of 78% (by volume) nitro-gen gas and 21% (by volume) oxygengas. The percent water vapor in theatmosphere is variable but may gener-ally range from a fraction of 1% up to4% (by volume). The solid or ‘‘frost’’noted on the can is water vapor con-densed directly to a solid. The liquiddroplets coming off of the paint cancontain liquid oxygen and have a nota-ble pale blue coloration. Oxygen gashas a boiling point of �183 8C andnitrogen gas has a boiling point of�196 8C. The liquid nitrogen in thepaint can cause the condensation ofthe oxygen gas in the atmosphere, thuddripping off the lowest point of theexterior of the can. It also likely con-tains some other liquefied gas (possiblyargon with a boiling point of �186 8C)since it is noted that the test tube con-tains immiscible liquids which vapor-ize rapidly.

The ignited puffed cotton (with thecollected condensed liquid poured onit) burns rapidly as compared tountreated puffed cotton. This can beused to initiate a discussion on theincrease in the rate of combustion dueto the increased concentration of oxidi-zer. It also emphasizes a safety concernwhen using cryogens like liquid nitro-gen as a coolant of a combustible mate-rial in the presence of air.14 This couldcause an explosive situation due to thecondensation of oxygen in the air andthe production of the powerfuloxidizer,liquid oxygen. An example is the use ofliquid nitrogen traps for vacuum pumpswhen any organic vapors are con-densed and then air is drawn throughthe trap. The flammability and explo-sive hazard potential when combusti-bles are in the presence of liquid oxygenshould be emphasized.

Figure 5. Liquid oxygen collectionapparatus.

22 Chemical Health & Safety, July/August 2004

Hydrogen–Oxygen ExplosionSafety Tips/Cleanup—The electrodeapparatus is removed from the 1.0 Mmagnesium sulfate solution (in theplastic container) when the demonstra-tion is complete. The lid is placed onthe plastic container and the solutionsaved for the next demonstration. Ifevaporation occurs, water is added tothe container. Always make sure thereis at least 1.0 cm of solution betweenthe surface of the liquid and the pipetbulb where the bubbles are released inthe apparatus. This is critical to insurethat the gas mixture trapped in thefunnel does not ignite. Always do thedemonstration behind a safety shield.Only ignite small bubbles. Do not allowlarge numbers of bubbles to accumulatebefore igniting them; otherwise theexplosion may be too violent.

Materials—Chemical splash goggles,laboratory coat or apron, gloves, safetyshield, clear circular plastic container(10–15 cm diameter) with lid, graphiterod and lead rod for electrodes, plasticcoated connecting wires, epoxy glue,large clear plastic funnel which fits intoplastic container when inverted, smallpipet bulb, magnesium sulfate, dish-washing detergent, nonfloating plat-form for electrode assembly, variabledirect current voltage supply.

Preparation—The apparatus may beconstructed similar to that displayed inFigure 6. The nonfloating electrodeplatform must be somewhat smallerthan the bottom of the plastic con-tainer and made of a weighted woodor plastic. Construct the electrodeassembly by inserting one graphiterod (cathode) and one lead rod(anode) into holes in the platform.Attach (solder) the plastic coated con-necting wires to the electrodes. Gluethe electrodes and connecting wires inplace with an epoxy glue so that theconnections are not exposed. Invertthe plastic funnel over the electrodeapparatus and secure with wire tothe electrode platform. Drill severalsmall holes in the lower portion ofthe inverted funnel to allow it to befilled with solution. Fill the plastic con-tainer with 1.0 M magnesium sulfatesolution containing 0.5% dishwashingdetergent. It may be necessary toexperiment with different dishwashingdetergents to find the one that gives the

best bubbles. Slowly place the elec-trode assembly into the plastic con-tainer, allowing it to fill with thesolution and finally rest at the bottomof the container. Cut the top of theinverted funnel so that it is at least1.0 cm below the surface of the solu-tion. Place and secure the pipet bulbonto the neck of the funnel. Cut a 1–2 mm hole in the top of the pipet bulb.

Demonstration—After setting up theapparatus as shown in Figure 7, con-

nect the wires to a variable powersupply and insure that the lead elec-trode is the anode (connected to thepositive terminal of the power supply).Place the apparatus behind a safetyshield. Apply a potential difference of8–12 volts and ask students to notetheir observations as to what is occur-ring. Allow bubbles to accumulate onthe surface for 30 seconds. Ignite thebubbles and ask students to note theirresults.

Figure 6. Hydrogen gas/oxygen gas production apparatus assembled electrode unitis shown—figure on the right.

Figure 7. Assembled hydrogen/oxygen gas explosion apparatus (power supply notshown).

Chemical Health & Safety, July/August 2004 23

Discussion—A popular lecturedemonstration is to ignite oxygen-filledballoons, hydrogen filled balloons, andlastly oxygen–hydrogen filled bal-loons.15 Hydrogen can create violentand loud explosions when placed inthe presence of a wide range of con-centrations of oxygen or air. The igni-tion of an oxygen–hydrogen filledballoon is not recommended fordemonstration due to the large explo-sion and excessive noise created. Thedemonstration described in this sec-tion produces small bubbles of an oxy-gen–hydrogen mixture which may beignited to display the smaller violentreaction but at a much safer noise level.The electrode reactions are as follows:

Cathode4H2O þ 4e� ! 2H2ðgÞ þ 4OH�

Anode 2H2O ! O2ðgÞ þ 4Hþ þ 4e�

Overall6H2O ! 2H2ðgÞ þ O2ðgÞ þ 4OH�

As the small bubbles are pushedthrough the small hole in the pipet bulb,theywill come to thesurface and maybeignited to give a more controlled explo-sion and noise level. The demonstrationmay be used as an inquiry-basedapproach to discussions chemical prin-ciples such as oxidation and reduction,electrolysis, and surface tension.

Chemical industries that use or pro-duce hydrogen gas take extraordinaryprecautions to avoid ignition sourcesand hence fires and explosions. Aircontaining between 4% and 74%hydrogen is explosive and air-hydro-

gen explosions have created muchdamage and loss of life.

CONCLUSIONDemonstrations enhance the ability ofstudents to understand scientific con-cepts including laboratory safety prin-ciples. Safety demonstrations, by theirvery nature, relate to hazards in thelaboratory. It is imperative that thedemonstrator practice the demonstra-tions several times in advance and takeall the necessary precautions. We feelthat efforts of incorporating demon-strations in our laboratory safetycourses have been fruitful in makingour students more ‘‘safety-conscious’’.

AcknowledgementsWe wish to acknowledge Hung

Chau, University of New South Wales,for his work in preparing some of thedemonstration apparatus.

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