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ExplodingSoapBubbles

Soapbubblesblownwithnaturalgasorhydrogenareignitedwithacandleastheyrisetowardtheceiling.

MATERIALS

soapbubblesolution

glasspipe

naturalgas(methane)orhydrogen

candle

matches

PROCEDURE

Soapbubblescanbemadebyfillingaglasspipewithasmallamountofsoapsolution(soldintoystores).Suitablesolutionscanalsobemadebyaddingafewdropsofsodiumorpotassiumoleateorliquiddishdetergenttoabeakerofwarm,distilledorverysoft,purewater.Asmallamountofglycerineandafewdropsofammoniawatergreatlyimprovesthelastingqualityofthebubbles[1].

Thepipeissupportedbyaringstandandconnectedtoasourceofcompressedgas.Agaslighterthanair,suchashelium,*

allowsthebubblestorisetotheceilingastheyarereleasedfromthepipe.Thegasshouldbeallowedtoflowthroughthepipebrieflybeforefillingitwithsoapsolutioninordertoexpeltheair.Ifthebubblesareblownwithhydrogen,theycanbeignitedwithacandleheldinthehandastheyrise[2,3].Someoxygencanbemixedwiththehydrogen,usingthearrangementshowninthefigure,tomakeamuchlouderexplosion.Theloudestexplosionwilloccurwhenthehydrogen-to-oxygenratiohasastoichiometricvalueof2:1soastoproducepureH2O.Naturalgas,oftenconvenientlyavailableonthelecturebench,makesaverybeautifulandquietflame.Naturalgasconsistsprimarilyofmethane(CH4)whichisslightlylessdensethanair.MixturesofmethaneandoxygenwillriseonlyiftheratioofCH4

toO2

islarge.Theeffectisbestifviewedagainstadarkbackgroundinsubduedlight.

*SpecialtygasesandequipmentcanbeprocuredfromMathesonGasProducts,P.O.Box1587,Secaucus,NJ07094.

DISCUSSION

Thisdemonstrationillustratesanumberofphysicalconcepts.Theveryexistenceofbubblesisadramaticdemonstrationofsurfacetension.Abubbleconsistsofathinfilmofliquid,heldtogetherbysurfacetension.Ithaselasticpropertiessimilartothoseofaballoon.Thesizeofthebubbleisdeterminedbythepressureofthegasinsideitandtheradiusandthicknessofthewaterfilm.Thatthegasinsideabubbleisunderpressurecanbeillustratedbyblowingabubblewithapipe,butbeforethebubbleisreleased,allowingittodeflateandblowoutacandlenearthestemofthepipe.Asmallerbubblehasahigherpressurethanalargerbubbleforthesamereasonthataballoonishardtoblowupatfirstbutbecomeseasierasitgetslarger.ThiscanbeillustratedwithaT-shapedpipeinwhichtwobubblesofdifferentsizeareblownandthenconnectedtogetherbywayofthepipe.Thelargerbubblewillgetlarger,andthesmallerbubblewillgetsmaller.

ThetendencyofabubbletorisewhenfilledwithagaslighterthanairillustratesArchimedes'principle.Thebubblewillriseifitsweightplustheweightofthegasinsideislessthantheweightoftheairdisplaced.Atroomtemperatureandatmosphericpressure,acubicmeterofairweighsalmostthreepounds.Abubblefilledwithairwillslowlydroptothefloorbecauseoftheweightofthefilmofwater.Theweightoftheairinsideapproximatelycancelstheweightoftheairdisplaced,neglectingthesmalldifferenceinpressure.Fromtheradiusofthebubbleanditsterminalvelocityasitfalls(seethediscussioninsection1.4),onecanestimatetheweightofthewaterandhencethethicknessofthewaterfilm(thedensityofwateris1000kg/m3).Anair-filledbubblecanbemadetofloatinabathofcarbondioxideproducedbydryiceinthebottomofatransparentcontainersuchasafishtank.

Bubblesareveryfascinatingandinstructive.Entirelecturescanbegivenonthepropertiesandbehaviorofbubbles,andsuchdemonstrations[4-7]inevitablyenthralltheaudience.

HAZARDS

Asidefromthenormalprecautionsinvolvedwithflames,theoxygenmixturescanmakequitealoudexplosion.Thedemonstratorshoulduseearplugsandcautionthemembersoftheaudiencetoholdtheirhandsovertheirears.

REFERENCES

1.C.L.Strong,ScientificAmerican

220,128(May1969).

2.B.Z.Shakhashiri,

ChemicalDemonstrations,TheUniversityofWisconsinPress:Madison,Wisconsin,Vol1(1983).

3.H.A.Robinson,ed.,

LectureDemonstrationsinPhysics,AmericanInstituteofPhysics:NewYork(1963).

4.C.V.Boys,

SoapBubblesandtheForcesWhichMouldThem,EducationalServicesIncorporated,DoubledayAnchorBooks:GardenCity,NewYork(1959).

5.C.Isenberg,

TheScienceofSoapFilmsandSoapBubbles,TiestoLtd.: