Science Buddies: "Ask an Expert"

Hello,

I am helping my daughter with a science experiment. We are comparing the densities of regular sodas and diet sodas by placing the cans in a large container of water. The expected result is that regular sodas sink while diet sodas float. You can see videos of this experiment all over the internet, all with this result. My problem is all my cans are floating - a sugar laden Peach Nectar, a regular coke, a coke zero, and a coke light - all are floating. Can someone help me explain this? Am I doing something wrong? Please help!

Thank you,

Anne McCaffrey

I'm sorry to hear about this odd result. The only thing I can think of would be the temperatures of the cans. My naive expectation would be that warming a can will cause it to expand, and therefore become less dense. Thus it is just possible that a cold can of Coke regular would sink but a warm one might float. (If you try to warm the sodas above room temperature be sure not too heat them too much or they will burst open, creating a mess. Of course, do not try to heat a can in a microwave; it won't work and could damage the microwave oven.) Were all the cans you tried at room temperature before you put them in the water? Also, the temperature of the water that was used to float the cans could be a factor. Cold water is denser than hot water, thus it is a tiny amount easier to float in cold water than in warm. (This is holds true for water above 4 degrees Celsius.)

I'll keep looking for an answer.

A very long post. Sorry I could not make it shorter.

I've searched on the Web for relevant information, and I learned two things:

1. As you know, when all the folks who put cans of regular ("Original" I think) Coke in water, the cans sank. This contradicts your experience. First hand data takes precedence over information found on the Web, so it is within the bounds of possibilities that the other sites that "explain" this experiment have not actually done the experiment, but just copied some other Web information. I have seen this happen. However, several sites have photographs of their submerged Coke can, so unless they have all faked their results to get the "right" answer (for example by adding a weight that can't be seen in the photograph -- a rather extreme hypothesis!!) their experience contradicts yours. We are left with a mystery. Could it possibly be that your can of regular Coke is not the kind of Coke with all the extra sugar? Check the ingredients table on the side of the can -- from Web discussions it seems that regular coke has something like 49 grams of sugar per can, so if your can shows only a small amount of sugar it may be a formulation that differs from "Classic" Coke. I am unfamiliar with the available kinds of Coke and their appearance since I do not drink sodas, so it's hard for me to tell how easy it would be to get the cans mixed up. If your can does indicate a lot of sugar, >10 grams say, then it must be a "regular" Coke and then we are faced with a flat-out contradiction (or the very extreme hypothesis put forth above! (or something I haven't thought of)).

I shall assume that you have already repeated your experiment several times with the same result.

As a scientist, your next step then should be to get someone you know to perform the test of whether regular Coke sinks or (as for you) floats. (They should buy their own can of regular Coke.) If they get a different result from yours, then you can zero in on exactly what is different in what they did in this test versus what you did.

If you both get the same result, contradicting the information on several Web sites, then you should attempt to contact the authors of the Web site content to see if they can throw any light on your experience.

If you find some major problem with the way this experiment is presented on the Web, your final step would be to write a paper and submit it to a science education journal.

I know that this train of action is extreme, and it is unlikely that you have the time and/or interest to pursue it. But that is what a scientist does "for real". Meanwhile you should present your data AS IS to the science fair, including if possible a live demonstration of a submerged can of regular Coke. Should make for a really interesting presentation!

2. It has been a very long time since I took a class in chemisty, but from reading on the Web, especially this URL

http://www.newton.dep.anl.gov/askasci/c ... m03838.htm

I have concluded that the explanations of why a solution of sugar in water is more dense than plain water given (at least tacitly) in these Web sites are all bogus. You might think this unlikely, but it would be far from the first case where a simplified "psuedo-explanation" given in text books and other educational contexts, is actually quite wrong. You can find a few examples at

http://www.ems.psu.edu/~fraser/BadScience.html

In truth, one cannot determine the density of a water/sugar solution except by actually measuring it (see the first URL given for the details). An example of a completely non-intuitive result is that adding 18 mL of pure ethanol to a liter of pure water a room temperature only increases the volume of the mixture by 14 mL! Four mL have "gone missing". Turns out that they are hidden in a sense within the molecular organization of the solution; the solvent allows the water molecules to pack together a little bit better than for the case of pure water. So the combination is less than the sum of its parts. See

http://en.wikipedia.org/wiki/Partial_molar_property

for more details.

Whew! This long post is over. Just remember, the truth is everything in science.

Sincerely,

John
Proud to be a Scientist

Hello AnneMcCaffrey,

I won't try to add to John Dreher's long discussion of possible problems. Following John's comments about real observations being the final word in science, I decided to make some observations.

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Experiment on the density of drinks in aluminum cans

Experimental Procedure
I went to my refrigerator. The only experimental subjects I could find there were a can of sugar-flavored ginger ale and a can of beer. I filled the sink with cold water and dropped in the cans. The first thing I noticed is that aluminum drink cans have a concave bottom, and air is trapped there if you drop the can in oriented as it would sit on a table. Be sure to tilt your cans as you immerse them to get rid of the air bubble.

Results
My result is that both my cans floated just as yours did. Both of them floated with water almost completely covering them, but they didn't sink to the bottom of the sink.

Conclusions
All drinks in cans are almost the same density as plain water. Small variations in how the cans were filled, their temperature, small variations in the can manufacturer and other factors are all approximately as large as any density differences due to sugar content. Immersing the cans in water is a very poor way to measure the density of the liquid.

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A hydrometer is a device for measuring the density of liquids dating back to about the fourth century AD. It may be said to have been used successfully a few more times than shown in any internet video. You can buy a hydrometer suitable for measuring the density of water and liquids of slightly greater density at http://www.amazon.com/Hydrometer-Triple ... 0064O94I0/ for $7.99 or at many other web merchants. Such a device will allow you to not only know if a liquid is lighter or heavier than water, but also by how much.

Take the liquid out of the can. Shake it and let it stand to get rid of any carbonation bubbles. Place some of the liquid in a tall slender container, drop in the hydrometer and read the hydrometer. You're done!

Hope this helps, WW

Thanks, WW, for your help. For medical reasons I am unable to move very far without assistance, so actually doing the experiment myself proved impossible for me. Your experience increases the chance that the Web "information" is bogus I regret to say, even though some of these "experiments" are described at educational Web sites.

This post is just for reference for anyone looking through the forums.

I was intrigued by these results, so I decided to test this on my own. I only had Pepsi (41 g of sugar) and Diet Pepsi ('0' g of sugar) on hand, so that's what I used. I tested room temperature cans in hot, room temperature, and cold water, and every single time, the Pepsi sank and the Diet Pepsi floated. I then also heated the cans and cooled other cans and placed them in room temperature water. I had the same results.

So, I think that the other results on this page may be due to the carbonation level in the soda. The density of a substance is changed by the amount of gas dissolved in it, so that may be what has been messing with those experiments.

-Meg