DENSITY on Dry... · that affect their science learning through high school. Some of these...

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The Science Teacher 46 46 46 46 46 DEMONSTRATIONS WITHOUT BUOYANCY CHALLENGE STUDENT MISCONCEPTIONS By Julie C. Libarkin, Cynthia D. Crockett, and Philip M. Sadler D ENSITY DRY LAND ON A s a property of matter, density is a topic central to secondary school physical science. However, student misconceptions about the concept abound. Through interviews and open-ended questionnaires we have found that middle school students hold a number of nonscientific ideas about density that affect their science learning through high school. Some of these nonsci- entific ideas can be traced to experiences with buoyancy in water. Standard density experi- ments involve objects floating and sinking in water (Rohrig 2001), and many students mistake buoyancy-related phenomena for characteristics of density. Alternative activities allow students to explore density with solid materials and may help dispel misconceptions.

Transcript of DENSITY on Dry... · that affect their science learning through high school. Some of these...

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DEMONSTRATIONS

WITHOUT BUOYANCY

CHALLENGE STUDENT

MISCONCEPTIONS

By Ju l ie C . L ibark in , Cynthia D . Crockett , and Phi l ip M. Sadler

DENSITYDRY LANDON

As a property of matter, density is a topic central to secondary school physicalscience. However, student misconceptions about the concept abound.Through interviews and open-ended questionnaires we have found thatmiddle school students hold a number of nonscientific ideas about densitythat affect their science learning through high school. Some of these nonsci-

entific ideas can be traced to experiences with buoyancy in water. Standard density experi-ments involve objects floating and sinking in water (Rohrig 2001), and many studentsmistake buoyancy-related phenomena for characteristics of density. Alternative activitiesallow students to explore density with solid materials and may help dispel misconceptions.

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Common student ideasMany density misunderstandings have been previouslyidentified in literature (Stepans et al. 1986; Klopfer etal. 1992; Kohn 1993; and Smith et al. 1997), but ourresearch suggests that secondary school students holdmisconceptions about the concept of density especiallyas it refers to the buoyancy, or floating and sinking, ofobjects in water. Density misconceptions are typicallyover-generalizations based on actual observations andcan be placed in three categories:

� Size;� Shape; and� Material.

Students with size misconceptions believe large ob-jects sink in water and small objects float. Studentswith shape misconceptions believe objects have buoy-ancy properties that are a function of shape, with con-comitant sorting in a water column based on shape.Students with material misconceptions believe thatbuoyancy behavior is a function of what a material ap-pears to be, regardless of the actual density of the mate-rial. For example, these students assume an object thatlooks like metal will behave like metal, and all metalswill exhibit the same density behavior.

Student responses to both open-ended questionsand interviews exposed each of these common mis-conceptions. Students were first asked to predictsinking and floating behaviors of balls of differentsizes (Figure 1). The majorityof students chose incorrect re-sponses, most commonly be-cause of the size of the ball(Figure 2, p. 48). Occasionally,students chose the correct re-sponse (Figure 1, response D)and still demonstrated size ormaterial misconceptions.

Size and shape misconcep-tions were particularly apparentwhen students were asked topredict and explain the behaviorof a number of objects in a bowlof water (Figure 3, p. 48).Rather than drawing the objectsat the bottom of the bowl, about70 percent of the students dis-tributed the objects based onsize and shape. Most of thesestudents drew the large cube atthe bottom, the pyramid in themiddle, and the small cube atthe surface. Students widelyvaried where they placed the

odd shape in their drawings; it was usually placed inthe lower half of the water column. Student explana-tions were dominated by the idea of size, where largerobjects always experienced “more” sinking than smallobjects. A secondary group of students also thoughtshape was the characteristic most closely related tosinking (Figure 2, p. 48).

Several factors other than density come into playwhen dealing with the two-state—water and solid—system. For instance, materials heavier than water canfloat on the surface because of surface tension, or ifformed into boat shapes. Water also varies in its densityif temperature or salinity is altered. For of all these rea-sons, we suggest that buoyancy is not the most effectiveway to teach density and propose the use of a one-statemodel—in this case solid-only—which may be morehelpful in improving student conceptual understanding.Single-state activities using liquids such as oil and waterhave been proposed, but these are typically time-consum-ing to prepare, potentially messy, and the buoyancy in-volved may also confuse students about density (Steinand Miller 1998). Buoyancy is not a simple function ofmaterial density, but is rather a function of the pressurecontrast between a reference column and the column be-ing observed. Therefore, a floating boat is not a simplefunction of the density of the boat’s material and alsodepends upon the volume of displaced water and thevolume and density of air existing below the water line.That said, students have difficulty conceptually differen-tiating density effects from buoyancy behavior.

Mary and Tony are in science class. They are using two balls made of the samematerial (which looks like wood). Mary puts the largest ball in the water and watches it sink:

After removing Mary’s ball, Tony puts the small ball into the water. What do you think happenswhen Tony puts the small ball into the water?

Circle the picture that you think best represents what will happen to the small ball.

Explain why you circled this picture.

F I G U R E 1

Forced-response question.Students were asked to predict sinking and floating behaviorsof balls of different sizes.

A B C D E

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F I G U R E 2

Student responses to questions (shown in Figures 1 and 3) about floating and sinking.

F I G U R E 3

Open-ended question.Students were asked to predict and explain thebehavior of different objects in a bowl of water.

The teacher walks by and hands Tony and Mary four objectsmade of the same material as the large and small balls:

Mary reminds Tony that the large ball sank earlier when theyput it in the water.

Draw each of the four objects in the water-filled containerbelow based on whether you think they will sink or float:

Explain why you have drawn the objects this way.

Solid activitiesUsing only solid materials in the activities describedbelow avoided the student misconception that waterwas necessary for density separation. (This view wasproposed by one of the students in our study who,when asked if solid materials in a bottle might sepa-rate if the bottle were shaken, responded, “No, youneed water!”) The solid materials had to have verydifferent densities for sorting based on density to oc-cur and to avoid issues of packing-related size sortingwhen additional objects were added. The followingsupplies were used in the activities:

� 450 g plastic or glass bottle with cap (clear spicebottles work well);

� Two sets of solid balls or beads of differentmaterial, but of same shape and size. Oneset should be low density (e.g., wood orplastic); the other set should be high density(e.g., metal, such as steel). Enough of eachtype of bead to fill the bottle one-half totwo-thirds full. If beads are used, both setsshould have similar structure—using dif-ferent types of beads causes students to usestructural differences (such as holes) to ex-plain density contrasts;

Figure

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Student response

A: “Because if you compare it to the big ball it will probably be light enough to float.”

E: “Because it will have more buoyancy.”

C: “It is roughly half the size of the large ball so it will float at about half the depth.”

D: “I think it will sink because it is round.”

C: “I think that it would float...since the object is made out of a material that looks like wood.”

A: “I did this because wood floats on water.”

D: “Because wood is heavy and the second object isn’t much smaller than the first.”

“Because bigger objects sink.”

“The square seems to weigh more so it would be at the bottom. The triangle seems to weigh a little lessso it would be higher than the square. The other items look light so they would float up at the top.”

“The small square is light so it stays at the top the large square is heavy so it sinks, the odd shapeis a little lighter so it will float a little.”

“Because the bigger the object the farther down it will go, and sometimes it depends on the shape.”

“I think the square and the triangle and the other shape will stay up because they are big anddifferent shaped but the small square will sink because it’s small.”

“I drew them like this because they are different shapes and sizes.”

Misconception

Size

Size

Size

Shape

Material

Material

Material/Size

Size

Shape

Size/Shape

Size/Shape

Size/Shape

Size/Shape

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� About three each of high- and low-density ballsor beads of different shapes and sizes, but thesame materials as in previous bullet point; and

� Two sets of balls or beads of a common material(such as plastic) and size, but different shapes.

Activity 1: Sorting of solid materialsThe first activity compared two materials of the sameshape and size to allow students to observe the separationof solid materials. We filled the bottle about two-thirdsof the way with stainless steel ball bearings and plasticbeads of the same shape and size, each about 0.5 cm indiameter, leaving enough room for separation to occur.Other materials, such as wood or plastic foam, also workbut the density contrast between the two materials mustbe great enough to allow separation to occur. For ex-ample, using two metals such as copper and lead wouldnot work because the densities are too similar.

The filled bottle was capped, held horizontally, and gen-tly shaken from side to side (Figure 4a). The plastic ballsseparated perfectly from the stainless steel balls (Figure 4b).This demonstrated that the materials were separating basedon some property although students were still not awarethat density, an intrinsic property of the material, was caus-ing the separation. Students tried to explain the demonstra-tion by saying that the metal was harder than the plastic, themetal was attracted to the bottom magnetically, or even thatthe difference in color between the two materials was affect-ing the separation. In order to effectively demonstrate theconcept of density, additional elements were needed.

As shown in Figure 2, students believed that large objectssink and small objects float in water, and materials behavedifferently as a function of shape. Therefore, studentsneeded to observe the behavior of objects of different sizeand shape. We chose objects about four times the volume of

the initial beads. Leaving the initial beads in the bottle, otherobjects were added one at a time, and students were askedto predict what would happen when the bottle was shaken.The majority of students thought the larger objects wouldbe at the bottom of the bottle. Other students thought theobjects would be side by side based on size, although theyweren’t sure where in the bottle the objects would come torest. Even students who adamantly claimed that sinking orfloating in water was a function of material or density wereunsure how these objects would behave outside of water. Asbefore, plastic objects separated to the top, with steel objectsbeneath, regardless of size and shape (Figure 5).

F I G U R E 4

a) Bottle filled with beads (yellow = plastic; red =stainless steel) of two different densities, priorto shaking.

b) Separation of beads by density occurs as aresult of shaking.

A

B

F I G U R E 5

Density separation of objects of varying shapes andsizes can also be illustrated with solids. Here, plasticobjects separate to the top, with steel objectsbeneath.

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Repetition of this demonstration with several differ-ent objects helped students focus on the point that objectsof like material behave in a similar fashion. Some sizesorting occurred, such that large objects tended to cometo rest at the top of the layer of identical material. Thiswas simply a function of packing wherein smaller objectsslipped into spaces below larger objects, essentially mov-ing and pushing larger objects toward the top. Because ofthis effect, if the two materials used were too close indensity, all of the larger objects, regardless of material,would end up on top, seemingly contradicting the con-cept of density. Using materials with a large density con-trast avoids this complication.

Activity 2: Shape doesn’t matterAlthough the first activity clearly showed that object be-havior is independent of shape and size, many studentsbelieved that additional sorting based on shape would oc-cur if enough objects of differing shape were added to thebottle. To combat this belief, we developed a second, verysimple activity. Two sets of beads—the same size andcomposition, but one set was spherical while the other wascubic—were put in a second bottle (Figure 6). Studentsagain predicted what they thought would happen whenthe bottle was gently shaken. Although all students deter-mined from the previous activity that shape did not affectseparation, many students still reverted to their initial mis-conceptions. For example, some thought the objects wouldlayer; students were evenly split on which type of beadwould be on top. Explanations for this layering includedgravitational attraction between the round beads and theEarth, the “pushing” of the round beads out of the way bythe square beads, and possible magnetism. Only a fewstudents, about 10 percent, declared that the beads wouldremain mixed after shaking. Of course, this was the case!After this demonstration the majority of students wereable to pinpoint composition, not shape or size, as thefactor most likely to determine sorting behavior. Addi-tionally, all students were convinced that water was not anecessary component of a density sorting experiment.

Although most students abandoned their misconcep-tions based on these two demonstrations, a few studentsthought we were tricking them. After shaking the bottlesfor themselves and observing layering in the first activityand mixing in the second, the hesitant students were alsoconvinced that separation was independent of size andshape. Further discussions with students led them to theconclusion that the separation they initially observed wasa property of the material. We were then able to continuewith a discussion of density as an intrinsic property.

Based on student comments throughout the activi-ties, we found this approach, including student predic-tion of object behavior and student manipulation of thebottles, to be an effective way to directly teach the con-cept of density. Additionally, the easy availability of theneeded materials makes this an ideal activity for theclassroom. After engaging secondary students in theseactivities, teachers interested in discussing buoyancy be-havior can do so without also combating common mis-conceptions about density.n

Julie C. Libarkin (e-mail: [email protected]) is aresearch associate, Cynthia D. Crockett (e-mail:[email protected]) is a research assistant, andPhilip M. Sadler (e-mail: [email protected]) isthe department head, all in the Science Education De-partment at the Harvard–Smithsonian Center for Astro-physics, 60 Garden Street MS-71, Cambridge, MA 02138.

Acknowledgments

We wish to thank our colleagues Harold Coyle, Anila Asghar, andFrancine Rodgers for their help with this research. Our entire team ofconsulting teachers helped us create and test our ideas in a classroomsetting. This work was supported by the DESIGNS project funded byNational Science Foundation (NSF) Materials Development GrantsESI-9452767 and ESI-9730469. An NSF Postdoctoral Fellowship inScience, Mathematics, Engineering, and Technology Education(DGE-9906479) supported Libarkin during the course of this study.

References

Klopfer, L.E., A.B. Champagne, and S.D. Chaiklin. 1992. The ubiqui-tous quantities: Explorations that inform the design of instructionon the physical properties of matter. Science Education 76:597–614.

Kohn, A.S. 1993. Preschoolers’ reasoning about density: Will itfloat? Child Development 64:1637–1650.

Rohrig, B. 2001. Making a mini-submarine. The Science Teacher68(2): 38–41.

Smith, C., D. Maclin, L. Grosslight, and H. Davis. 1997. Teachingfor understanding: A study of students’ preinstruction theoriesof matter and a comparison of two approaches to teaching aboutmatter and density. Cognition and Instruction 15(3): 317–393.

Stein, M., and D. Miller. 1998. Density explorations. The ScienceTeacher 65(2): 45–47.

Stepans, J.I., R.E. Beiswenger, and S. Dyche. 1986. Misconceptionsdie hard. The Science Teacher 53(6): 65–69.

F I G U R E 6

Shape has little control overmaterial sorting.