

Gummy Bears and Density
Name: Colin H.
Status: educator
Grade: 68
Location: CO
Country: USA
Date: Winter 20092010
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 38 "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:
1the mass was incorrectly measured (less than true)
2the volume was incorrectly measured (more than true)
3there was a mistake in calculating density (unlikely)
4the bear sinks for another reason (an unexplained downward force
or a lessening of the buoyancy force)
5we 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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Update: June 2012

