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Name: Colin H.
Status: educator
Grade: 6-8
Location: CO
Country: USA
Date: Winter 2009-2010


Question:
Why does a gummy bear that has soaked in water have a density less than water, but not float in that water? We did the "soak a gummy bear" lab and calculate the change in density. The density does down to below 1g/cc, but none of the bears were floating. What is the explanation?



Replies:
I am confident that "gummy bears" do not violate the Archimedian principle of buoyancy, so we have to look for a different explanation. This is a good experiment because it requires looking at our assumptions and/or measurements one at a time. What are the different steps? (I am not playing games with you, because I do not know the answer. The answer is what we are seeking. )

What can we measure with our most confidence? The "dry" weight of gummy bear.

We can know the volume of the "gummy bear" by weighing it in a liquid where there is little or no possible interaction between the "gummy bear" and the liquid. Choose any liquid whose density is well known and that has no interaction with "gummy bears". A choice might be isopropanol (available at any hardware store). This is a good selection because you have a "double check". The density of isopropanol compared to water you can find in any number of handbooks. The "solubility" of a "gummy bear" in isopropanol should be negligible. So, depending upon this result (that is, "gummy bears" are more dense than isopropanol (I am assuming that, I do not know the answer.)

Since the mass of the "gummy bear" (probably??) does not change, maybe the volume of the wet "gummy bear" does increase -- which would factor would be more important? I do not know. Is it even in the right direction. That is for you to determine. But that is what science is about -- discovering the unknown.

Now you have to keep searching. How did you decide that the density of "gummy bears" was less than water? The scientific principle: Always examine your assumptions.

I cannot give you "the answer". I do not know it. But it is the process of learning that is central to science. By the way use 3-8 "gummy bears". Otherwise, you will not know if the results only apply to your particular choice of the bear.

Vince Calder


Colin,

I am afraid I do not know the Gummy Bear Lab, but I assume you soak the bear and then measure its mass and volume and then calculate the density.

If its density becomes less than 1g/cm3 it should float (you already know this!!!) in water, but you observe that it does not. So, our possible resolutions are:

1-the mass was incorrectly measured (less than true)

2-the volume was incorrectly measured (more than true)

3-there was a mistake in calculating density (unlikely)

4-the bear sinks for another reason (an unexplained downward force or a lessening of the buoyancy force)

5-we have a new observation that refutes our models of buoyancy force and density

These all need to be considered, but my hunch is with number 2. How did you measure the volume? If you did three length measurements and multiplied them together you will probably get a larger volume than the true, lessening the density you calculate, giving you the impression that it should float. You could try a displacement method (maybe involving multiple bears to give a large enough displacement to make errors less important) like the supposed Eureka! Bath tub moment.

In my opinion, this is a wonderful experiment and outcome for students to really learn about science and, most importantly, students should examine possibility 5 and not discount it because everyone "knows" the laws of buoyancy.

By the way, it is extremely unlikely, in my opinion, that the density of the bear could get below that of pure water by soaking it in water. I assume it starts off denser than water (sugar certainly is) and I would expect the density to asymptotically approach that of water as more water is mixed with the contents of the bear.

Best wishes,

Tom Collins


Colin

Your calculation must be off. It happens to scientists and engineers all of the time. Plus if the gummy bear has been in the water for long, some of the sugar might have dissolved in the water.

At 4 degrees C (39.2 degrees F), water density is 1000 Kg/cubic meter.

See:

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

Be sure you get accurate measurements of the weight and volume of the gummy bear.

Sincere regards,

Mike Stewart



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