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).


Frank Noschese February 26, 2010 at 6:38 pm

8-pound bowling balls also float! Very dramatic.


Dallas Raby February 27, 2010 at 1:15 am

I remember this discussion in school over 60 years ago. It had to do with why Ivory soap is the only soap that floats. I’ve liked Ivory soap for that reason ever since. I remember the principle, but I forget why it floats. I think maybe air is whipped into it.

sciencegeekgirl February 28, 2010 at 1:40 am

Yup, Dal, you remember correctly. Most soap will sink. Ivory was billed as “so pure it floats.” Ivory was actually made by mistake, when the operator left it stirring too long and it whipped it full of air. They realized that this was a way to use less soap in each bar. Silly consumers. We’re so easily marketable.

So, yes, Ivory floats because it’s less dense than usual soap, because of the air whipped into it.

And try putting Ivory into the microwave for a cool trick.

Matt Kriebel March 1, 2010 at 2:20 am

I hate to burst a great story, but:


Mind you, this is only recent revelation, comparitively speaking.

Although it is true that lighter soap is lighter on the ingredients demand.

sciencegeekgirl March 1, 2010 at 3:09 am

Doh! Foiled by Snopes. Too bad. So many great stories, ruined by the truth. Thanks, Matt.

Rich Whitehouse March 5, 2010 at 9:03 am

sgg…I think you may be off on your assertion that the floating soda can demo was a bit of a lie. The cans will indeed float or sink based on density relative to the density of the fluid in which it is placed. It seems you took into account only the density of the soda in your assumption. In fact, you must take into account the density of the entire contents of the can, which includes any gas bubbles. Density is, after all, a ratio of the total mass of an object to its total volume. The difference between the diet soda and regular soda would be the difference in mass. Would could assume the volume would be essentially equal.

Now, if I am missing some really obvious point, ignore me and I will go away. That said, I do wish to tell you that I enjoy your site.

Have a great day.


sciencegeekgirl March 5, 2010 at 3:52 pm


It’s only a “lie” in that it doesn’t *only* depend on the density of the soda (diet vs regular). Sure, at the end of the day, it depends on the density including any trapped air at the top of the can. But that’s not what teachers usually use it to teach. So sometimes the can may be sinking or floating due to differences in trapped air, rather than due to the idea that is meant to be explained by the activity — the difference in density of the soda itself. Whether you call it a lie, or just complicated, is a matter of semantics!

Taylor March 9, 2010 at 1:07 am

I am supposed to write two hypothesies for the same soda can experiment…The first one I have is: The drink with more mass sank,and the can with the least mass floated. What should my second hypothesis be? NOTE: I am in 5th Grade…..So, 5th grade level hypothesis!! write back soon!
My second question is: Suppose that type of drink did not affect which can floated or sank. Maybe the cans themselves were different in some way. Maybe something besides soda got into one of the cans by mistake. Write down at least two possible factors that could have caused the events.
My third Question is: Write down any other possible explainations you can think of. Could the cooler have had any affect? Could something in the water be responsible? Could there be something in the water you couldn’t see?
My fourth Question is: Review your answers to question 1. Use one of your ideas to write a hypothesis explaining why one floated and one sank. (Hint: Try to use the words If…..Then….)
Question 5. Review your answers to questions 2 and 3. Choose one of your statements describing something besides the type of drink that caused the floating or sinking. Write a hypothesis based on that idea.
Question 6. Are both of your hypthesis testable? Write a breif description of how you could test each one. Mention any equipment you would need. (Hint: You can open the cans and pour out the drinks as part of your tests.)
Question 7. Review your work. Use it to help you write a short summary of hhow to develop a hypothesis about an event. (MY SCIENCE HOMEWORK) PLEASE HELP As Soon As Possible!

sciencegeekgirl March 9, 2010 at 1:26 am


Good initiative in searching on the internet for assistance with your homework. But it’s not right to ask someone else to do it for you! The blog post should help you a little bit.

Think about things you have experience with and sinking and floating. You said that the one with more mass should sink. Test your hypothesis first with something easy. Soap is a nice one. Take a bar of soap. Does it sink or float? What about if you break it in half, do you expect it will sink or that it will float? Try it. How might you use this to understand the soda? Make a good guess at what you think will happen, and why, and find out a way to test it. It’s not too hard — you probably do this all the time in places you don’t think about it.

Taylor March 9, 2010 at 1:50 am

Thanks SGG, the blog helped me with number One. I am currently on number five. Your tips could help me out one day…..Your blog is a life-saver! I wasn’t asking for you to do it….I was asking for tips. I read your theory a few times and I am starting to click with the idea alot better. You could make a great Science teacher one day! 🙂

Taylor March 9, 2010 at 2:00 am

A bar of soap will sink. (except Ivory)
The pieces will sink also. Actually, This is my first time dealing with hypothesis and thats what im confused with…..I know what to put,I just can’t word it. The experiment is simple, the hypothesis statements……(not so much)in fact, i have ALWAYS been interested in sinking and floating since first grade! I ? science! 🙂

~ ?Taylor?

sciencegeekgirl March 9, 2010 at 2:01 am

So, what’s something that could make it sink or float other than the type of drink? I can think of a few. Keep up the good work!

Taylor March 9, 2010 at 2:01 am

those ? are supposed to be hearts…. sorry

Taylor March 9, 2010 at 2:05 am

Thanks SGG I got second place in my school science Fair….My experiment was “Paper Towel Testing” I was testing the strength of paper towels. I used different brands to determine the strength and absorbency with the number of pennies it would hold before breaking. 🙂

sciencegeekgirl March 9, 2010 at 4:06 am

Hi Taylor,

A “hypothesis” is just a fancy word for “what I think will happen.” Like, if I think that my dog eats likes Brand A dog food better than Brand X then I might say,
“My hypothesis is that my dog will eat more Brand A than Brand X.” That’s not a great hypothesis because I’m not sure how to test it — what does “more” mean? So maybe I’d make it better by saying, “My hypothesis is that my dog will eat a greater volume of Brand A than Brand X.” Looks like your teacher wants you to use the words “if” and “then” in your hypothesis, so he/she wants you to phrase it as a cause and effect. Like “If I feed my dog Brand A, then he will eat more food than if I feed him Brand X.”

Mathew Anderson April 16, 2010 at 1:36 am

I really like your no BS style, very rare in the academia ! Keep up the good work, I will book mark for regular entertainment. Let us know if you fancy a guest blog post.

Alivn May 13, 2010 at 7:58 pm

This is for my science experiment.THanks on the story on this

Jim Richardson October 8, 2010 at 1:49 am

This is very interesting. I’m already a college graduate and going for an online MBA program and honestly, I didn’t know about some of the things that you said until I read it here. I’ve been trying to guess what will happen to the particular object as I read and found myself wrong. I do believe that this is a great way to teach kids. They are very curious and I believe they will enjoy fun activities like this. Those are amazing teaching tips! I’m glad that I came across your site, I’ll be happy to read more interesting articles from. I’ll just help myself here and browse along.

Peter Lyons May 23, 2011 at 9:45 pm

Here’s a cool one you might like with soda cans and metronomes.


it January 6, 2013 at 1:01 am

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