In the Classroom
The Return of the Black Box Malka Yayon and Zahava Scherz* Department of Science Teaching, Weizmann Institute of Science, Rehovot 76100, Israel; *
[email protected] “If protons, quarks, and other elementary particles are too small to be seen, how do scientists know they exist? And if these particles do exist, how can one estimate their size, structure, and or their arrangement in atoms?” These are some of the most frequently asked questions by students who study atomic theory. Atomic structure is an important topic and we feel that all students should be exposed to it. However, the concepts involved are abstract and difficult for students to comprehend. It is also difficult for high-school chemistry teachers to provide meaningful answers to the questions their students ask (1–4). Some teachers have found that using analogies drawn from students’ daily experiences or thoughts can help students understand certain science concepts. Others teach the historical development of atomic theory, progressing from Dalton to Bohr, to provide a more realistic view of science (5–7). We believe that models and the process of modeling are fundamental aspects of science and that curricula should provide students with meaningful modeling activities. The “black box” is an example of an activity in which students are asked to make indirect observations of an object and to try and deduce some of the properties that characterize it. The activity offers a good opportunity for students to experience the scientific method: collecting and analyzing evidence, constructing a model based on this evidence, using the model to explain and predict new phenomena, revising and evaluating the model, and constructing a revised model if necessary (8). In the classic black box activity students are asked to describe the contents of a sealed container by making a series of external observations and tests, such as shaking and tilting, without opening the box, purely on the basis of secondary evidence. In other cases the students identify an object according to a scattering pattern that is generated while they throw marbles at a covered object (9) or determine the “plumbing system” of a “magic water” black box with tubes protruding out of it by pouring water into the inlet tubes; different inlet tubes result in different outlet colored water (10). In both activities the students are asked to provide as much information as they can about the contents of the box without opening it. These activities give the students some idea of the methods used to deduce the model of the atom (9–14). In practice, students enjoy the activity, but they become disappointed because the information they obtain, using these simple methods, is not particularly interesting, and most of the experiments that are suggested to deduce the content of the box are either not practical or not available (e.g., X-ray analysis). We felt that we could improve the black box by creating an activity in which students could apply a step-by-step procedure similar to that used in the development of the scientific model in general and the atomic model in particular. During the activity the students become aware of the connection between the tool used and the nature of the results it produces. The more the model is probed with various tools, the more data are available to revise the model, making it a more accurate representation of the real thing. One should always keep in mind that there might be something that is impossible
to detect and it is important to remember that the model is never the real thing. In the following paragraphs we describe an activity with a black box. The reader is invited to follow each step of the black box activity and to create a model of the contents of the box. Detailed directions for how to make black box kits can be found in the online supplement. The students’ answers are examples of comments given during the activity. A Step-by-Step Description of the Activity The students are told that this activity is analogous to the research that enabled scientists to develop a model of the structure of the atom, in which particles that are too small to be seen were investigated. Stage 1 Students are given a sealed box and are asked whether, in their opinion, the box is empty. They are also asked to suggest experiments needed to support their answer. We used a video cassette box wrapped in grid paper, as shown in Figure 1. Specific areas on the box can be described by a combination of a letter and a number. For example, the combination E1 refers to the area in the upper right side of the container. The students know that this is a video cassette box. Usually, some students say that the box is empty or that it contains only air because they do not hear any objects rattling inside. Other students suggest that there may be something that is attached to the inside of the box. The students are asked to propose ways to resolve this issue. In some cases they propose to compare the weight of the box to that of an empty box. If they do not come up with any idea, the teacher may show an empty video cassette box as a clue.
Figure 1. A video cassette box wrapped with grid paper.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 4 April 2008 • Journal of Chemical Education
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In the Classroom
Figure 3. X-ray image taken from the short side of the box.
Figure 2. X-ray image taken from the wide side of the box.
Conclusion After weighing both boxes, it becomes clear that the box is not empty, and the students proceed to stage 2. Stage 2 Students are asked to suggest experiments to learn more about the contents of the box and its structure. Usually the students readily think of X-rays. If not, the teacher can prompt them by asking what can be done if a doctor wants to determine whether you have a broken leg or how you can check whether somebody is carrying a bomb into an airport. Conclusion An X-ray of the box should be performed. Stage 3 An X-ray photograph of the box is given to the students (Figure 2). They are asked to describe, using the X-ray photograph, what they can and cannot conclude about the contents of the box and its structure, and to think of new experiments that can be done. Conclusions There are at least three objects made of dense materials that are detected by the X-rays: they are cylinders, discs, or spheres and there must be something else that holds them in place so they do not move freely while the box is shaken. In order to decide whether the objects are cylinders, discs, or spheres, a new X-ray should be taken from another position. Stage 4 A new X-ray image of the box is given to the students (Figure 3). The students are again asked to describe the information that can be deduced about the contents of the box and about the shape of the objects. They are asked to think of experiments that can determine the materials of the objects seen in the X-rays. Conclusions There are at least three “flat” cylinders made of materials that can be detected by the X-rays. There must be something else made of a material that does not absorb X-rays that holds the cylinders in place, since at least two of the flat cylinders are in the middle of the box. 542
Students are told by the teacher that the flat cylinders can be made of metals, ceramic, or bone. Since the flat cylinders do not look like bones, students typically assume they are made of metal and think of ways to detect metals. They usually suggest testing for metals by using a magnet. The teacher states that there are only three common metals that are attracted to magnets: iron, nickel, or cobalt (ferromagnetic materials). All the other metals are not attracted to magnets. Stage 5 The students are asked to scan the entire surface of the box with a magnet (we will refer to it as the “scanning magnet”). While moving the scanning magnet over the entire surface of the box, it becomes obvious that there are two interesting zones. In one of them, both poles of the magnet are attracted to the box (B4), and in the other, one pole of the scanning magnet is attracted whereas the other pole is repelled (E4). Conclusions There are at least three objects made of dense materials that are detected by the X-rays. These objects are discs stuck in a material that seems to fill part or the whole box because they do not move while the box is being shaken; at least two of them are in the middle of the box. One of the flat cylinders (B4), is made of a metal that is attracted to a magnet (iron, nickel, or cobalt) since the attraction between the object and the scanning magnet was independent of the orientation of the scanning magnet. The other flat cylinder is a magnet, since holding the scanning magnet in one direction results in repulsion, while holding it in the other direction results in attraction (E4). The third flat cylinder is composed of a dense material that is not attracted by a magnet (E2). As concluded previously, there must be something else made of a material that does not absorb X-rays that holds them in place, since at least two of the flat cylinders are in the middle of the box. Stage 6 At this point, students are asked to draw a model of the contents of the box while specifying the number of objects, the location, shape, materials, and so forth. We found that at this stage, students are able to create models in which they describe the contents of the box, also taking into consideration objects that were not detected by any of the experiments. Stage 7 If the model suggested is correct, it should be able to predict the results of new experiments. The students are asked to predict what would be observed when iron sawdust is spread on the box.
Journal of Chemical Education • Vol. 85 No. 4 April 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Classroom
upon the model they built. The students come to understand that observation leads to a model, the model leads to further experiments, and these experiments consequently result in an improved model. The more information accumulated, the better the conclusions and predictions obtained. This activity emphasizes the importance of the relationship between the tool being used and the information gathered, and last but not least, the importance of being aware of the information that was not obtained using a certain method. Acknowledgments Figure 4. Contents of the black box: a magnet, an iron cylinder, two coins (which seem to be one cylinder), a plastic disc (which was not detected in any of the experiments), a sponge with slots, and adhesive tape. Note that the photo shows an arbitrary arrangement of the contents.
To test their hypothesis, students are given a sealed petri dish filled with half a teaspoon of iron sawdust. The iron sawdust is arranged in a round shape near the magnet (E4), as expected. This step is rewarding to the students who went through the whole process as it represents an important property of a scientific model: the ability to predict to new phenomena. During the activity the students gathered a lot of information about objects that could not be directly observed, and any properties assigned to the objects were inferred by interpreting the results of the experiments. However, there is a technique that enables people to directly observe the objects in the box and that requires breaking it. In the case of the atom, you do not get a chance to “see” what is there. But breaking the box could be analogous to gathering information while accelerating and colliding particles; these particles break apart, giving scientists a clue of the structure of the unbroken particle. The next step is dramatic for the students; the teacher shows the actual components of the “broken” box, starting with the known parts: a magnet, an iron cylinder, two coins (which seem to be one cylinder), a plastic disc (which was not detected in any of the experiments), a sponge with slots, and adhesive tape (Figure 4). At this stage the students can reconstruct the box using all the information gathered during the activity and see how their observations are related to the box’s actual structure. They realize that they are not able to place the plastic disc in the box, since they lack the necessary evidence. It is important to reflect and think about the investigative process and relate it to the scientific method in general and to the scientific method used to deduce the model of the atom in particular. Concluding Remarks This activity provides a meaningful learning activity for students. They gather information using multiple “instruments”, organize the information, and draw conclusions that strengthen or modify their model. They also can make predictions based
We wish to thank Hannah Margel and Ronit Halevy for fruitful discussions and Ruth Ben Zvi and David Fortus for helpful comments. Literature Cited 1. Ben-Zvi, R.; Eylon, B.; Silberstein, J. J. Chem. Educ. 1986, 63, 64–66. 2. Ben-Zvi, R.; Eylon, B.; Silberstein, J. J. Educ. in Chem. 1988, 25, 89–92. 3. Margel, H.; Eylon, B.; Scherz, Z. J. Chem. Educ. 2004, 81, 558–564. 4. Harrison, A.; Treagust, D. Sci Educ. 2000, 84, 352–381. 5. Hohman, J. J. Chem. Educ. 1998, 75, 1578–1579. 6. Smith, R. J. Chem. Educ. 1989, 66, 637. 7. Goh, N. K.; Chia, L. S.; Tan, D. J. Chem. Educ. 1994, 71, 733. 8. Fortus, D.; Hug, B.; Krajcik, J.; Kuhn, L.; McNeill, K.; Reiser, B.; Reiser, B.; Rivet, A.; Rogat, A.; Schwarz, C.; Shwartz, Y. Sequencing and Supporting Complex Scientific Inquiry Practices in Instructional Materials for Middle School Students. Presented at the Annual Meeting of the National Association for Research in Science Teaching, San Francisco, CA, April, 2006. 9. Records, R. J. Chem. Educ. 1982, 59, 307. 10. Chien, A.; Godshall, A. Magic Water: A Black Box Activity. Summer Research Program for Science Teachers. http://www. scienceteacherprogram.org/genscience/Chien05Lesson/INDEX. HTM (accessed Dec 2007). 11. Clement, J. Int. J. Sci. Educ. 2000, 22, 1041–1053. 12. Holton G. The Project Physics Course: Text and Handbook; Holt, Reinhart and Winston: New York, 1970; Unit 5. 13. McClellan, A. Chemistry; An Experimental Science: Teachers Guide; Freeman: San Francisco, 1965. 14. Schaff, J. Semimicro Experiments for the Chem Study Program: Teacher’s Guide; Heath: Boston, 1966. upporting JCE Online Material S http://www.jce.divched.org/Journal/Issues/2008/Apr/abs541.html Abstract and keywords Full text (PDF) Links to cited URLs and JCE articles
Color figures
Supplement
Student handouts
Directions and a list of materials to make the black box
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