Floating Lemon Sinking Limes
Scientists seem to be infatuated with objects that float and sink. Even non-
scientists find great joy in dropping stuff in water to see if it floats or sinks.
Fans of David Letterman are quick to point out one of Dave's favorite
segments called "Will It Float?" Here's the latest float or sink challenge: Why
do lemons float in water but limes sink? Think you know the answer? Not so
quick...
Materials
Take a trip to your local grocery store in search of few lemons and limes that
are roughly the same size and weight. A big lemon and a tiny lime will throw
off your test results. You'll also need a deep container like a large bowl to
conduct the float and sink test.
Experiment
1.Fill a bowl with water and drop in the lemons. Float or sink? They should
float.
2.Toss the limes into the water. Float or sink? You might get a few that float
but just barely. The limes in our test sank to the bottom.
3.Now it's time to figure out why. Your first guess might be the rinds of both
fruits. Peel the rind off of the lemon and you'll find that it is thick and porous “
similar to the rind of an orange (that also floats in water). The lime rind is
much thinner than the lemon and does not contain the same porous material.
4.It's time to test the peeled lemon and lime. Drop both in the water and you
might be surprised. The lemon floats and the lime sinks! So, it's not the rind.
5.That's as far as we're going to take you. Keep experimenting and exploring
before reading any further.
How does it work?
At first, our team of lemon lime researchers were certain that it had everything
to do with the rind because of their experience with floating and sinking
oranges. Drop an orange into water and it floats, but remove the peel from that
same orange and it will sink. The unpeeled orange floats because the rind is
very porous and filled with tiny pockets of air. Even though you're removing
mass when you peel the orange, the peeled orange is more dense and sinks
in the water. But the lemon lime mystery is a little different.
We have to turn to the Internet for more information. According to the USDA
website, a lime is 88.26% water by weight and a lemon is 87.4%. This could
mean that lemons have a higher air content, but we're still not sure.
So, our next step was to weigh the lemon and the lime. Digging through our
grocery bag of lemons and limes, we found two that were very similar in size
and weight. Both the lemon and the lime weighed exactly 101 grams. If you
know the weight, it only makes sense to determine the volume of each fruit.
Using displacement of water, we determined that that the lemon had a
volume of 99 mL and the lime had a volume of 90 mL. You might remember
doing this in school when you learned about volume. Fill a container to the
very top with water and then submerge the item (the lemon in this case).
Collect and measure whatever water spills out over the edge of the container
and you've got the volume.
So, using the density formula (D=M/V) it was easy to determine the density of
each fruit.
The Lemon “ Density =101g/99 mL = 1.02 g/mL
The Lime “ Density = 101g/90 mL = 1.12 g/mL
Since the density of water is approximately 1 g/mL, it makes sense the at the
lemon in our experiment floated and the lime sank.
But we're still a little confused as to why limes sink and lemons float. Some
people have suggested that lime flesh is much more dense than lemon flesh,
and as a result it holds its juice better. People who "juice" fruits know about
this phenomenon. If you want to get more juice from a lime, warm it up in the
microwave briefly and then firmly roll it along the work-top before slicing it
length-ways and juicing it. It makes a huge difference.
Since the density of the lemon and lime are very close to water, even very
small changes in composition could mean the difference between sinking
and floating.
Experiment from Steve Spangler Science
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