science corner - march 1996

buoy oh buoy - a science experiment students CAN do

questions are the beating heart of science. Knowing how to phrase a good question is an art that is developed only after phrasing many poor ones. Everything from basketball lay-ups to writing calligraphy requires practice. This month student notice that some cans of soda pop float in water and others sink. It will take quite a few questions and a chunk of time to isolate the variable that might be responsible for this phenomena. Because students are testing variables, this is a true experiment. Most Science Corner ideas involve students with interesting activities not experiments because there are no controls involved. This experience allows students to practice the hallmark of what Piaget called Formal Operational thinking. They phrase thoughts with this mental construct: "if - then - therefore." Instead of using the Learning Cycle model, this lesson is process oriented.

To determine some of the variables responsible for cans of soda pop sinking or floating.

Getting Started:
purchase a variety of soda pop - perhaps 20 cans, making sure there is a mix of brand names, generic soda and even some non-carbonated drinks like ice tea and lemonade. Invite students to bring a can of soda or juice for tomorrow's science lesson to increase the variety and number of drinks examined. Each group of 3-4 students will need some type of basin to test their cans. A two quart juice container makes a nice "testing pond." Again, one student in each group might be responsible for bringing a container. Do not tell them the nature of the lesson. Let them wonder overnight.

diet sodas float and regular sodas sink. Usually. Start off with a can of Pepsi that has been tested to be a "sinker." Holding it over a transparent container so students can see the results, ask: "What might happen to this can when I gently place it in the water?" Asking, "what MIGHT happen," invites students to participate. It is a totally different question than posing, "what WILL happen." This requires either previous knowledge or clairvoyance.

now repeat the question with the Diet Pepsi that has been determined to be a "floater." Continue to place other cans in, asking students to speculate on the floating fate of each. Let them experiment in their groups. Although they will enjoy all the comparisons, it will soon be obvious that they need a way to systematically test the variables. Ask them to design a chart to keep track of the concepts they test. Do not present the one below --- let them struggle with the process of how to gather, organize, examine, and interpret the data they collect. Making a chart (can't do it on Netscape) -- but you would have categories such as those below --- is quite a challenge and learning experience.

variables to consider

diet drinks vs. non-diet drinks
liquid volume: what it says on the can and what is in the can (ml)
can mass: aluminum or iron ?

variable might be a new term to some students but all of them know its synonym: "no fair." Students often scream "no fair" during their school day if someone gets a head start in a race; or more dessert in the cafeteria; or an easier class assignment. Ask students to substitute the word, 'variable,' when they think that any comparison is not valid or "no fair." Now the new term, variable, is used in the context of something they have just experienced.

here are variables students suggest might be responsible in some degree for cans being sinkers or floaters.

even though all the cans may say 354 ml or 355 ml - do they contain that? Measure their contents. By chance, students might find one that has 320 ml or 340 ml. The machine at the factory turned off a little too fast in the filling process. These cans float significantly higher than others.

below is a chart showing the mass of 60 cans. Note the remarkable consistency among the average mass : 389 g for Sprite; 388 g for Coke and Pepsi.

Diet drinks are about 20 grams lighter in each case; 372 g for Diet Coke and 362 g for Diet Pepsi.

For reference, a nickel is minted to have a mass of 5.00 g. The 27 g difference between the Diet Pepsi and Sprite equals more than five nickels.

isolating variables and designing a way to test them is a difficult task and student only become skilled at it through practice.

Hypothesis: stating a problem to be solved as a question that can be tested.
are the liquid volumes of all the cans identical ? Do sodas with caffeine have a greater mass than the same brand name that is caffeine-free? Do all the empty, rinsed cans have the same mass? What 'test' has to be performed to rule out that . . . . "caffeine" content is the determining factor ? (test must be with same brand name; same type: diet or non-diet / one is caffeine free and the other contains caffeine.)

some of their hypotheses would be impossible to test. For example:
Do cans with white paint have a greater mass than the same can with orange paint?
Still, their raising such issues and thinking about how they could possibly be tested is part of the learning process.

perhaps the most important variable is the drink's sugar content. Diet drinks contain NutraSweet whose mass is much less than the sucrose of regular drinks. Boil away equal quantities of a diet and a non-diet drinks in an aluminum pie tin and compare the residue. Few elementary school science supply closets have a hot plate but this allows teachers to enlist the aid of the cafeteria staff who will supervise the boiling process on their stove. The liquid can be boiled away directly or using a double boiler method. The former results in 'charred remains' that look like sugar burned in the oven when the cherry pie spills over. The double boiler process results in a deposit of cola syrup. Have students prepare a computer-generated "badge" for the cafeteria staff:

Laboratory Technician
4th grade Science Helper

this will greatly enhance the rapport and respect between the students and the school's all-important support staff.

While overseas I have collected diet colas that SINK.

After students have gone through what they deemed a valid scientific process that "proved" the presence or absence of sugar was a major the variable responsible for floating/sinking. I place a diet drink into the water and it sinks. Students are stunned. The Piagetian term is "mental disequilibrium." They have to start thinking all over again and eventually discover a new variable -- the can. In many parts of the world they are still using iron-based containers. A magnet determines this rather quickly.

if a 'sinking diet' or 'floating regular' drink can be found before the activity begins, do not let students see it until they have come to the opposite conclusion. Then show them the oddity. This will cause them to change their conclusion to say, "Most diet drinks float and most regular drinks sink."

Pooling Thoughts
this experiment is not intended to explain why things float or sink. That involves a discussion of buoyancy; density; displacement of water, etc. This lesson gives students experiences with float-sink phenomena that will make its "formal explanation" understandable at a more developmentally appropriate time -- like the junior high school. What students learn about variables will be important in other curriculum areas. Learning how to test for the role of each will immerse them in thought although the exercise may be mentally draining.

science sleuths & subterfuge
tell students that sinkers can be "taught" to float. Rub a "sinker" while whispering "coaching instructions" to it. Place the can against the aquarium / container wall or into a corner so it is perpendicular to the water. This traps an air bubble under the can and it should float. Practice these 'swimming lessons' before class. Students will be amazed. Remove the can and challenge them to make floaters out of the sinkers.

is it possible to reverse the process and turn a floater into a sinker? Yes, but there is a bit of deception involved here, too. Hide a small piece of clay in one hand and take a "floater" from the aquarium and discreetly press the plasticene to the bottom of the can. When lifting the can from the depths, remove the clay without being detected.

while "trickery" SHOULD NOT be used as a common practice in teaching science, the purpose of this subterfuge is to eng