The Cartesian Diver

The Cartesian Diver

CHEMISTRY/PART 4 ACTIVITY 2 The Cartesian Diver Purpose stretcha rubber ballon over the mouth of each con- Toinvestigate how salinity and pressure affect lainer and secure it with a rubber band. densityand how this affects objects floating in water. Pressthe sidesof thetwo-liter bottle containing the freshvvater. In the caseof the othercontainers, EquipmentINIaterials pressdown on the rubbercover. What happens to the drop peP *2 plastictwo-liter soda containers, 2 large test tubes Releasethe pressureon the bottle or rubber 0 x 3.5cm!, 2 graduatedcylinders 00 mlor cover.What happens to the dropper? larger! or 2 glassjars Whathappens to theamount of waterin the drop- 2 identicalmedicine droppers Cartesian Divers! per/testtube as it submerges? salt Now press on the sides or rubber cover of the teaspoon saltwatercontainer. Does the medicinedropper plastic funnel behavedifferently in the salty water? 'If largetest tubes, graduated cylinders or jars are Questions used,two piecesof thin rubber ballons!and two rub- ber bands are needed. t. Doesthe CartesianDiver submerge easier in fresh or salt water? Procedure 2. Does the Cartesian Diver nseto the surfacefaster in Fill each of the selectedcontainers almost full of fresh or salt water? water.Use the plasticfunnel to add 4 to8 teaspoons of saltto oneof the containers. Larger amounts of salt 3. What makes the CartesianDiver dive? shouldbe addedto the largercontainers.! Shake the containerto help the salt dissolve. 4. Why does the CartesianDiver rise back to the sur- Suspenda medicinedropper vertically in eachof face of the water? thecontainers. They can be suspendedwith a few drops of water in them, 5, Discussthe relationshipamong density, salinity and If you are using the two-literbottles, screw the pressure. caps on. If you are using any of the other containers, C/45 CHEMISTRY/PART 4 ACTIViTY 3 ScubaDiving and the Gas Laws introduction to Gas Laws Tointroduce the gas laws in the context of scuba Boyfe's Law dtvlrlg. Boyle'slaw statesthat at a constanttemperature, the volumeof a gas will vary inverselywith pressure, Background while the density of the gas varies directlywith the Peoplecan explore the marine world by using a pressure. mask,snorkel and fins and holding their breath. Qr This means that as the pressure on a volume of theycan prolong their exploration with the use of gas doublesthe gas is compressedto one-halfits scuba selfcontained underwater breathing appa- volume see Figure 2!. If the pressuretriples then the volumeis compressedto equal one-thirdof its original ratus! equipment, Theself-contained breathing apparatus consists volume. ofa tankfilled with compressed air,a regulatorwith a mouthpieceand a BCD Buoyancy Control Device! Figure1!, Figure 2 Boyle'slaw pressure Volume Density Lungs of a diver Depth on lungs of air of air holdingbreath Figure1 Self-containedbreathing apparatus 0 feet 1.0 atm 40 liters 1 kg/f BCD 33 feet 2.0 atm 2 0 liters 2 kg/1 /2! regutator A regulatorreduces the high pressure ofthe air in 66 feet 3.0 atm 4/3 liters 3 kg/1 a scubatank to a usablelevel and deliversair when neededby the diver,A BCDis mandatoryequipment for all diving.It is an irrllatablesack that increases buoyancy.It is usedto providesurface support to rest, swimand maintainneutral buoyancy under water. Othertypes of divingequipment are available but are lessaccessible to the generalpublic. Scubadiving has become a popularhobby for 99 feet 4.0 atm 10 liter 4 kg/1 manypeople. It offersa wealthof informationon div- /4! ing physiologyand the effectsof gas exchange. C/46 CHEMISTRY/PART 4 Oneof the first effects a diverfeels is increasing blockshis nose and attempts to exhalethrough rtwith pressure.As he descends,the pressurearound him hismouth closed. This forces air into the air spaces in- increasesby 1 atmospherefor every33 feetof de- sidethe ears and sinuses and relieves the squeeze. scentin saltwater 4 feetin freshwater! Figure 2!. Thistechnique isknown as equalizing. Theincreasing pressure is notfelt by boneand solid Theprocess of equalizingreturns the air spaces tissuebut by air spaces ears, lungs and sinuses! to theirnormal volume by equalizingthe external Figure3!. waterpressure and the pressurein the air spaces,As longas a diverbreathes compressed airand equalizes the pressurein the air spaces,he will notfeel the Figurie3 Air spaces in the body squeezing as he descends. Figure4 Techniqueto relievepressure on theear drum airw nasal c middle ear ngs stomach intestines Whena diverdescends, the increase in pressure reducesthe size of hisair spaces Figure 2! andcom- A diverascending experiences the opposite pressesthe air inside.A diver who holds his breath pressure-volumerelationship. The pressuredecreases feefsthe increase in pressure decrease in volume! as andthe volumeincreases. A divernot breathingon a squeezingin his lungs, sinuses and ears Figure 4!. ascentwill feel his lungs expand Figure 5!. A diver's Asthe pressure increases, the squeezing increases lungscan only expand 15 to 30percent of theirorig- andbecomes uncomfortable. Eventually the diver will inalvolume without bursting. Therefore, ascending stop his descent. withoutbreathing may result in lung tissue bursting Torelieve the pressureon the lungs,a diver Figure 5!. breathescompressed airat a regularfrequency. To relievethe pressure on earsand sinuses, a diver C/47 CHEMISTRY/PART 4 Figuris5Lungs ofascending scuba diver ascending scuba diver ascendingscuba diver breathing constantly holding breath surface RUPTUREi 33 feet expanding air 66 feet compressedair 99 feet temperaturesdecrease thetemperature ofthe gas in A smartdiver breathes onascent. Breathing thetank and 'the gas decreases involume. resultsinequalizing thepressure inside the lungs and Consequently,thetank will not have as much air airspaces withthe outside pressure sothat the normal init asexpected. This will shorten the dive time. To lungand air space volume ismaintained Figure5!, avoidthis problem, dive shops place the tanks incool Asthe pressure increases, thedensity ofthe gas waterduring filling so maximum airvolume isachieved. increasesdirectly. This means that if thepressure ona gasdoubles, thedensity doubles. Asthe pressure of thegas increases, remember thevolume decreases Dalton's Law Thisdecrease involume results inthe gas rnolecules Dalton'slaw states that the total pressure exerted occupyinga smaller space. This decrease inspace bya mixtureofgases isequal tothe sum of the occupiedbya setnumber ofgas molecules means pressuresofeach of the different gases making up thatthe density increases. Anincrease inthe density themixture each gas acting as if it alonewas pres- ofair makes it more difficult tobreathe atdeeper ent and occupiedthe totalvolume. depthsthan at thesurface Thislaw is illustrated inFigure 6, which shows air tanksfilled under three different conditions. Charles' Law Condition A Charles''law states that if thepressure is kept con- Thetank contains only oxygen molecules ata low stant,the volume ofa gaswili vary directly with pressure.Ifthe total pressure ofthe gas is 160 mm of temperature. Thismeans that as the temperature ofa gasin- Hg mercury!andoxygen makes up 100 percent of creases,the volume of spaceit occupiesincreases. thegas, what isthe partial pressure ofoxygen? Thisphenomenon isseen in a diver'scompressed air answer:160 mm! tank.Filling the tank warms the air and causes it to A diver'scompressed airtank is neverfilled with pureoxygen because it is toxic When using pure oxy- occupya greater volume, Under water, the cooler gen,the blood's hemoglobin becomes saturated with C t 48 CHEMISTRY/PART 4 Condition C Figure 8 Dalton'slaw Thetank's total pressure has doubled to 1,520 condition A condition B condition C mmHg atm!.The percentage of oxygen and nitro- gen in the tankshas not changed.But more of both gaseshas been added to increasethe pressure. What isthe partial pressure of oxygenand nitrogen> Poz = P»,i x o%%dOz PNz- P~ x i%%dNz = 1520 rnm x 0,21 - 'I,520 x 0.79 = 320 rnm = 1,200 mm Thepercent of each gas in the total gas mixture does not change. But as pressureincreases, each of the partialpressures increase by the same amount. If you compare conditionsB and C, you will see that the totalpressure has doubled and so haveeach of the 160 rnm Hg 760 mm Hg 1520mm Hg partial pressures. total pressure total pressure total pressure NitrogenGas rrndWater Preesure- oxygen.This saturationturns off the reflexescontrol- Nitrogen Narcosis lingcarbon dioxide exhalation and allows it to buildup in thebody, This results in nausea,vomiting, dizziness, Theair you inhale is a mixtureof 78percent nitro- gen, 21 percentoxygen, 0,03 percent carbon dioxide tunne!vision, blackouts and grand mal seizures. andsmall amounts of othergases, The oxygen in the Condition B air you exhaledecreases from 21 to 16percent and thecarbon dioxide increases from 0.03 to 5.6percent. Nitrogenhas been added to the tankso that the Duringexhalation, the body produces roughly the totalpressure, Pr, is 760mm Hg atm!.No oxygen sameamount of carbon dioxide as the oxygentaken hasescaped so the Poz is unchanged.What is the in. Nitrogenremains the same PNz? At the surface,the pressureof air is one atm or 760mm Hg. Nitrogenmakes up 78percent of the air P»~- Poz+PNz witha partialpressure of about600 mm Hg. Asa diver 760 mrn =160 mrn+ PNz experiencesgreater pressures, the partialpressure of PNz-600 mm nitrogenincreases the percentage of nitrogenin air remainsthe samewith increasing pressure!. At greater Whatpercentage of the totalpressure is contributed than 100feet

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