Will it sink or float? Soda cans and density.

by Stephanie Chasteen on February 26, 2010

Many teachers know the value of finding those surprising science experiments and demonstrations that hook kids’ attention.  One popular one is to have kids predict whether soda cans will sink or float, which turns out to be a nice hook for ideas of density.  Kids generally figure that if one thing of a kind sinks or floats, so will all the others.  So when the regular soda sinks, most will predict that all the other ones will sink too.  Much to their surprise, the diet soda generally floats.  Why?  Regular sodas are loaded with sugar, and that increases the density of the soda.  Diet sodas have aspartame, and a lot less of it per volume (take a look at the side of the can).  Steve Spangler’s site has a nice description of this activity.

I want to say more about the soda can activity (and why it is, in a way, a bit of a lie!) but first a few asides:

– This is, of course, also a great lesson in why sugared sodas are so bad for you.  That’s a lot of sugar.  Another astounding experiment I saw in this regards was to have kids weigh a piece of sugared gum, like Bazooka.  Then chew it, while the teacher talks about density and weight and stuff.  Then weigh the gum again.  It is surprisingly lighter.  Where’d all the weight go?  Look and see how many grams of sugar are in the gum.  That’s now in your belly.

– Kids have a lot of trouble with density.  They often think that “light” things float and “heavy” things sink.  Take a piece of soap and show that it sinks.  Break it in half and ask them to predict whether it will sink or float.  Many will predict that it will float, and will be visibly astounded when it still sinks.

– Another great density experiment is to take a piece of aluminum foil.  When it’s flat, it sinks.  When it’s crumpled up, it holds air, and floats.  More info here.

– Lastly, I’m forever enamored with the work of Dan Schwartz, who has kids invent the solution (for example, the formula for density) before telling it to them.  He calls these invention activities “Preparation for Future Learning” and there’s a lot of evidence to show that they’re effective.  For example, for density, he shows kids a bunch of cars with clowns in them and asks them to come up with a “crowded clown index.”  The index has to differentiate between, for example, the small cars with many clowns and the small cars with few clowns, as well as a large car with few clowns and a small car with few clowns.  Even if they don’t come up with the standard formula for density, students are ready to hear the expert solution, and also understand why density is a useful construct.  This goes along with the idea of giving a need for a vocabulary word before introducing the word itself.

OK, so now for why the soda can trick is (sort of) a lie.  It doesn’t always work.  It’s important to test the cans before you do this as a classroom activity (unless you want to turn it into an investigation of ‘why didn’t we see what we expected?’).  There is some variability in how sodas are canned, both within and across brands.  Is the advertised volume actually in the can?  (The only way to know is to open the can, though you can also weigh the can, as long as the same mass of aluminum is used).   Sometimes there might be extra air in the can, turning what should be a “sinker” into a “floater.”     Also, sometimes a bubble can get trapped under the can (so tip it sideways).   The temperature of the water also changes its density, so conceivably the temperature of the water could change the outcome of the experiment, though I’d be surprised if this was a large effect.

So, it’s a bit of a “lie” because you never know, perhaps you chose a floater and a sinker that float and sink because of different amounts of trapped air in the can, rather than because of density.  One could imagine turning it into an inquiry experiment, where students try to confirm the teacher’s hypothesis that the floating and sinking is due to density differences — a simple weight and volume determination of the soda in the can could do the trick, and would be a great experiment for students to suggest.  After all, don’t believe it just because teacher said so!

Image from Ngchikit under CC Share Alike (more info here).

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