Instructor Information
JCE Classroom Activity: #84
Whatever Floats (or Sinks) Your Can Michael J. Sanger Dept. of Chemistry, Middle Tennessee State University, Murfreesboro, TN 37132;
[email protected] In this Activity, students test whether cans of carbonated beverages sink or float in water and then determine whether caffeine content, soda color, or sugar content in the carbonated sodas is responsible for the buoyancy of the sealed cans. Soda cans filled with diet carbonated beverages float in water while soda cans filled with sweetened carbonated beverages sink in water. The difference in the densities of sweetened versus diet sodas is well known (1–4), although students should be reminded that this Activity investigates the density of the cans filled with soda and not the density of the sodas themselves. In addition to teaching students the importance of controlling variables when making comparisons, this lesson can also introduce students to the concept of buoyancy and the principle of flotation (5). Archimedes’ principle says that an object can displace a volume of liquid equal in mass to the object’s mass. An object that is denser than the liquid has a mass that is larger than the mass of the same volume of liquid. Although this object has enough mass to displace a volume of liquid larger than its own volume, once the object has displaced a volume of liquid equal to its own volume, it sinks to the bottom of the container. On the other hand, an object that is less dense than the liquid has a mass that is smaller than the mass of the same volume of liquid. So, the object will only be able to displace a volume of liquid equal in mass to its own mass. Since the object does not have enough mass to displace a volume of liquid equal to its own volume, only part of the object is submerged in the liquid and the rest remains above the liquid surface (i.e., the object floats). This leads to a general rule of flotation: Objects more dense than a liquid will sink to the bottom of the liquid, and objects less dense than a liquid will float in the liquid.
Integrating the Activity into Your Curriculum This Activity can be used as an introduction to density in a middle school physical science course, or a high school chemistry or physics course. It can also be used in elementary or middle school courses as an experiment in controlling variables and making meaningful comparisons.
About the Activity Although every student could test every can, this lesson lends itself well to class cooperation. Each group tests the buoyancy of one or two cans and shares the results. Each can should be tested by more than one group and any contradictory results addressed. If students use a container where the water level is not much taller than the cans, they may have difficulty distinguishing between floating and sinking cans. While most of the carbonated diet soda cans float very well, some of the sweetened carbonated soda cans have densities rather close to the density of water. This means they may sink very slowly, or appear to float. To be sure that students see that the sweetened soda cans sink, they are told to quickly push downward on any floating cans. Diet soda cans should quickly rebound and remain floating; any “floating” sweetened soda cans should remain submerged.
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Answers to Questions 1. It’s a matter of density. Objects more dense than water sink, while objects less dense than water float. 2. The sugar content is responsible for whether a soda can sinks or floats in water. Direct comparisons between cans can be used to identify the relevant factor. For example, since Pepsi and Caffeine-Free Pepsi sink, you can rule out the effect of caffeine; and since Diet Pepsi and Diet 7-Up float, you can rule out the effect of color. The effect of sugar can be seen since Pepsi sinks but Diet Pepsi floats. 3. Since Crystal Pepsi had sugar in it, we would expect a soda can of it to sink in water. 4. If you look at the labels of two sodas that differ only by sugar content (e.g., Diet Pepsi and Pepsi), you will notice that Pepsi has 41 grams of sugar in it that Diet Pepsi doesn’t have. The Diet Pepsi can is less dense than water and it floats. But, adding 41 grams of sugar to Pepsi makes the can more dense than water and it sinks. 5. You could open the cans and test the liquids themselves. If a few drops of a sweetened soda (like Coke or Sunkist) are added to water, the soda would sink as it mixes with the water. When a few drops of a diet soda are added to water, it floats on top as it mixes with the water. Dark soda work best for these tests.
This Classroom Activity may be reproduced for use in the subscriber’s
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Background
References, Additional Related Activities, and Demonstrations 1. Herrick, Richard S.; Nestor, Lisa P.; Benedetto, David A. Using Data Pooling To Measure the Density of Sodas: An Introductory Discovery Experiment. J. Chem. Educ. 1999, 76, 1411–1413. 2. Lovely, Gary; Runyan, Tom. 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. 3. Mitchell, Tony. Questions from a Can of Pepsi. J. Chem. Educ. 1988, 65, 1070. 4. Checkai, Gary; Whitsett, John. Density Demonstrations Using Diet Soft Drinks. J. Chem. Educ. 1986, 63, 515. 5. Hewitt, Paul G. Conceptual Physics, 8th ed.; Addison–Wesley: Reading, MA, 1998; pp 219–224. JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 83 No. 11 November 2006 •
Journal of Chemical Education
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JCE Classroom Activity: #84
Student Activity
Whatever Floats (or Sinks) Your Can
Try This You will need: container tall enough so that a can of soda can sink to the bottom (trash can, bucket, 1000-mL beaker, etc.), water, unopened cans of carbonated soda with different caffeine content, soda color, and sugar content (for example, 7-Up, Diet 7-Up, Caffeine Free Diet Pepsi, Diet Pepsi, Caffeine-Free Pepsi, Pepsi, Sunkist, etc.). You may use different samples of sodas, but the more cans you test the better your results will be. Each student could work individually and test every soda can, but this Activity works better when the class works together. Each group of students should test one or two soda cans and report their results on the board. Each soda should be tested by more than one group, and any discrepancies should be retested to reach an agreement. __1. Make a table in your notebook with five columns: Soda name, Caffeine content (“yes” or “no”), Soda color (“colorless”, “yellow–orange”, “brown”, etc.), Sugar content (“yes” or “no”), and Buoyancy test (“sink” or “float”). __2. Fill a large container with water. Be sure to leave enough empty space in the container so the water will not overflow when a soda can is added to the container. __3. Get a can of carbonated soda. Without opening it, determine whether the soda contains caffeine or sugar. The sugar content is listed in the Nutritional Facts label; artificial sweeteners would be a “no” for sugar content. If a soda contains caffeine, it will be listed in the ingredients or on the label. If you do not know the color of the soda, ask your partner, another student in class, or the instructor. Record the name, caffeine content, color, and sugar content of the can in your table from step 1. __4. Slowly place the can in the water to avoid splashing. Most carbonated soda cans have a concave bottom that could trap an air bubble. When placing the can in the water, tilt it slightly so you don’t trap an air bubble underneath the can. Once you have released the can, decide whether it is floating or has sunk. If the can is floating, quickly push on the top of the can so it is temporarily submerged. If it floats back to the surface, record that the can floats; otherwise record that it sinks. __5. Repeat steps 3 and 4 for any additional cans you will be testing. __6. Once you have tested your cans, write your information on the board. If another group has already recorded results for your soda, do not write your results on the board unless they are different. If they are different, find that group and work together until you agree on all of the information (caffeine content, soda color, sugar content, and buoyancy test). __7. Once all of the groups have reported their data and all discrepancies have been addressed, analyze the data to determine which property (caffeine content, soda color, or sugar content) is responsible for whether the cans float or sink.
Questions 1. Why do some things float in water while other things sink in water? 2. Which of the three factors (caffeine content, soda color, or sugar content) is responsible for whether a soda can sinks or floats in water? How do you know? 3. In 1992, Pepsi made a soda called Crystal Pepsi. It was carbonated, colorless, and contained sugar and caffeine. Would you expect a soda can of Crystal Pepsi to sink or float in water? Why? 4. Why does changing the sugar content in a soda change its buoyancy? 5. This Activity investigates the density of soda cans (including the soda, the aluminum can, and any trapped gases). How could you test the densities of just the sodas?
Information from the World Wide Web (accessed Sep 2006) Density Demonstrations. http://outreach.rice.edu/~dgabby/science/denisty_demo.htm How Liquid Motion Lamps Work. http://www.howstuffworks.com/lava-lamp.htm This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education •
Vol. 83 No. 11 November 2006 •
www.JCE.DivCHED.org
photos by J. Maynard and R. Wildman
If you’ve ever gone to a summer picnic that had a large container filled with carbonated sodas in ice water, you may have noticed that some of the cans were floating on the top with the ice while others were down at the bottom of the container. Why do some of the sealed soda cans float while others sink? In this Activity, you will investigate what makes the carbonated soda cans sink or float. You will look at three properties—caffeine content, soda color, and sugar content.
