

Melting Ice, Energy Transfer, Buoyancy, and Surface Area
Name: Elizabeth
Status: Student
Grade: 68
Location: OH
Country: United States
Date: November 2008
Question:
We made five ice shapes with the same amount of water in
each to see which shape would keep the water in which it was placed
the coldest for the longest. The shapes were pyramid with a surface
area of 175.31cm squared, a cylinder whose surface area was 150.72cm
squared, a rectangular prism whose surface area was 127cm squared, a
sphere with a surface area of 122.66cm squared, and a cone with a
surface area of 114.55cm squared. The amount of water and the
temperature of the water in which the shapes were placed were the
same for each trial.
Room temperature was also the same each time.
The results were all over the place. The pyramid (largest surface
area) did cool the most and stay the coolest. However, the cone
(smallest surface area) stayed cooler than the other shapes with
larger surface area. Also, the sphere and the rectangular prism
whose surface area were very close to each other had very different
outcomes. Where did I go wrong? What should the results have been?
Replies:
Elizabeth,
First, kudos to you for performing experiments of your own. Experiments are
a critical (and fun!) part of science.
This experiment is a great way for you to get a better understanding of how
heat transfer works. There are lots of possible reasons for your odd
results. If you can learn a few basic concepts related to heat transfer, it
would help you understand 'what results would be expected'. Explaining them
here would make for a very long reply (and this reply is already way too
long!), so I am going to get you started with a few concepts that you can
read about on your own or with the help of your teacher:
1. Conduction  this is one of the most important ways heat is transferred
from one material to another. Keep in mind that heat flows from hotter
objects to colder ones.
2. Heat capacity  this is how much heat it takes to change the temperature
of an object.
3. heat of fusion  this is how much heat it takes for a 'phase change'
(e.g. 'melting' from solid to liquid)
4. Convection  along with conduction, this is a second way heat is
transferred.
If you have any questions about these feel free to email me / AAS back.
Now for some comments:
First, your problem statement describes two different things  the
temperature of the water *and* the time it is at that temperature. It is
better to have a more precise problem statement  such as how long it takes
for the water to reach a temperature of "X", or at what temperature is the
water at time "X". In general, it is better to have a quantitative,
measurable hypothesis.
Second, you were right to realize surface area matters, but you have to keep
in mind some of the ice is not submerged (it is touching air), and the volume
that is submerged is changing as the ice melts. I am assuming the ice is
freely floating in the water.
Off the top of my head, there are lots of other factors that you might
consider 
1. water temperature is not the same everywhere in your container  the water next
to the ice will be colder than water far from it. Depending on where in the
container you measure temperature, you could get very different results. If you mix
the water well, you might get more reliable results  but be careful about
how much you do this because it will affect the rate of melting.
2. time  in the very short term, things like the initial surface roughness
of the ice are more important than how much ice there is... and in the long
term, shape will not matter at all (you will really just be measuring how fast
heat is moved from the waterice combination to the surroundings). It might
help to have a more defined problem  for example, how long it takes for
wellmixed water around the ice to reach a certain temperature.
3. Equipment  what are you using to measure temperature? Scientific
equipment is going to give a lot more reliable results; home thermometers
are probably not the best choice for this. You need a thermometer that
quickly and accurately measures temperature.
4. Details details details! You say that air temp and water temp are "the
same", but you do not say what they were. Did you assume they were 'close' or
did you measure them explicitly? If you did not, you should record these
values in your experiment. Record humidity too, as this will affect results.
Last, make sure there are no drafts/heating ducts/sunlight around the
container as changes in these things will all affect results.
5. More details! Based on the values you provide, I was not able to
reversecalculate the dimensions of your ice shapes. I could not find values
that fit the required volume and surface area. I want to make sure one of us
has not made a mistake. I suggest you go back and check your math on the
surface area and volume (feel free to email me back to compare
calculations). It is possible I (or you) made a mistake in math, or perhaps
your shapes are not the same volume after all  which would obviously also
impact the results.
p.s. One more topic you might want to read about: "significant figures" (or
significant digits). This is something most students do not learn about until
high school physics, but it is something that is important for scientists to
understand. Especially for #5 above, it matters in this experiment as well!
Hope this helps,
Burr Zimmerman
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Update: June 2012

