In the Classroom edited by
Erica K. Jacobsen The Dalles, OR 97058
JCE Classroom Activity #108. Using Archimedes' Principle To Explain Floating and Sinking Cans Michael J. Sanger Department of Chemistry, Middle Tennessee State University, Murfreesboro, Tennessee 37132, United States
[email protected] Instructor Information In this activity, students measure the mass of several 12-ounce diet and regular soda cans. Then, the students place the cans in water (noting whether they sink or float) and collect and then weigh the water displaced by the soda cans. Then, they compare these two mass values for the sinking cans and for the floating cans as a way to explain Archimedes' principle. Background In this activity, students should discover that the soda cans that sink in water have a mass greater than the mass of water they displace while the soda cans that float in water displace a mass of water roughly equal to the mass of the soda cans (usually within (1 or 2 g). This classroom activity is an adaptation of a longer laboratory previously published in this Journal (1). These results can be explained by students using Archimedes' Principle and can help them derive the concept of flotation (2). Lovely and Runyan (3) wrote an experiment using soda cans, spring scales, and salt water to teach students about Archimedes' principle, and Sutterby (4) described an experiment using an analytical balance, a heating mantle, and a smooth bob to determine the density of liquids at various temperatures using Archimedes' principle. Archimedes' principle states that an object can displace a volume of liquid equal in mass to the mass of the object. For an object denser than water, the object weighs more than the same volume of water. Although this object could displace more water than it does, once it displaces a volume of water equal to its own volume it sinks. Sinking objects weigh more than the mass of water they displace. An object that is less dense than water, on the other hand, weighs less than the same volume of water. So, a floating object cannot displace enough water to become fully submerged, only part of the object is submerged in the water, and the object floats. These results are summarized in the general rule of flotation: Objects more dense than a liquid will sink in the liquid, and objects less dense than the liquid will float (5).
or even a high school chemistry or physics course. This experiment works well with 12-ounce soda cans; it has not been tested with the smaller 8-ounce mini cans. This activity describes the use of an overflow can to measure the volume of water displaced by the soda cans. An overflow can is a small aluminum “beaker” with a small metal spigot/tube a few inches from the top of the can that allows the water to drain out of the overflow can without flowing over the top of the device (Figures 1 and 2). This is a common piece of equipment used in most physical science classrooms and can be purchased from scientific supply companies.
Figure 1. Materials for measuring the volume of displaced water (glass beaker, aluminum overflow container, and unopened soda can).
Integrating the Activity into Your Curriculum This activity can be used after the initial instruction on density as a way to introduce Archimedes' principle. Students should know how to calculate densities using masses and volumes and that floating objects are less dense than the liquid while sinking objects are more dense than the liquid. A similar experiment using soda cans, published as a previous classroom activity (5) in this Journal, can be used before this experiment to introduce students to these concepts of density. This activity can be used in a middle school or high school physical science course, 272
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Figure 2. Collecting the displaced water as the soda can is slowly lowered into the overflow container.
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Vol. 88 No. 3 March 2011 pubs.acs.org/jchemeduc r 2011 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100861h Published on Web 01/11/2011
In the Classroom
About the Activity In this activity, students will determine whether several different cans of carbonated sodas sink or float in water, and will measure the mass of the soda can and the mass of the water that it displaced. Each student (or group of students) can test every can, but this experiment will go faster without significant loss of learning if each student or group of students collects data for one or two different soda cans and pools their data by writing their results on the board. It is a good idea for each soda can to be tested by more than one group, and the instructor should pay attention to the results on the board to see that there are no contradictions among groups. If there are two groups that have tested the same sample of soda but come up with different results, the two groups of students should work together to come up with a consensus. The class should test at least three diet soda cans and three regular soda cans. It has been noted that while all diet sodas float very well and most cans of sweetened sodas sink in water, some cans of sweetened sodas will actually float in water (1, 6). This will not affect the results of this study. After all of the students have written their results on the board and all discrepancies have been eliminated, they are asked to compare the mass of the soda cans to the mass of water displaced by the soda cans by calculating a mass difference, Δm = m(can) - m(water). For the floating soda cans, these values should be the same (Δm = 0); however, there is always some error in the students' measurements, so the students should be reminded that these values should be “roughly equal” (-2 g e Δm e 2 g). From the previous study (1), we found that these masses are usually within 1-2 g of each other. For the sinking soda cans, the mass of the soda can should be greater than the mass of water
r 2011 American Chemical Society and Division of Chemical Education, Inc.
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it displaced; however, these values are more variable;the soda can mass can be as much as 10-15 g more than the displaced water or these values can be almost the same. While the students are writing their data on the board, the instructor should monitor these data to see that the two masses are roughly equal for the floating cans and that the masses of the sinking cans are more than the mass of the water it displaced. If there appears to be a fair number of data points that do not follow these general trends, the instructor can have students determine and report average values of Δm for the sinking and the floating cans. Literature Cited 1. Sanger, M. J.; Humphreys, T. C.; LaPorte, M. M. Using Soda Cans To Teach Physical Science Students about Density. J. Chem. Educ. 2009, 86, 209–211. 2. Hewitt, P. G. Conceptual Physics, 8th ed.; Addison-Wesley: Reading, MA, 1998; pp 219-224. 3. Lovely, G.; Runyan, T. Floating Cans. In Teaching Physics with Toys; Taylor, B. A. P., Poth, J., Portman, D. J., Eds.; McGraw Hill: Middletown, OH, 1995; pp 233-237. 4. Sutterby, J. L. Density by Archimedes' Principle Using a Top Loading Balance. J. Chem. Educ. 1976, 53, 249. 5. Sanger, M. J. Whatever Floats (or Sinks) Your Can. J. Chem. Educ. 2006, 83, 1632A–1632B. 6. Jacobsen, E. K.; Paulson, D. R.; Sanger, M. J. Soda Can Density and Unexpected Results. J. Chem. Educ. 2008, 85, 18–19.
Supporting Information Available Student activity worksheet; answers to questions for students. This material is available via the Internet at http://pubs.acs.org.
pubs.acs.org/jchemeduc
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Vol. 88 No. 3 March 2011
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