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Knowing When Material is Melted
Name: Annaliese
Status: student
Grade: 6-8
Country: USA
Date: Winter 2011-2012
Question:
Hi, for my science fair project I am asking: does the fat content in ice cream change the melting speed? I am not sure how you would test if the ice cream was melted
Replies:
Annaliese
I would suggest laying the lump of ice cream on a flat surface and measuring
the time it takes for all of the lumps to melt.
Be sure that the temperature of the flat surface (and the ambient air?) is
the same for all measurements.
Sincere regards,
Mike Stewart
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Annaliese,
As you have already figured out, it is unreliable to use some sort
of visual inspection for the melting of ice cream, or, for that
matter, any real world substance. The problem is that during
melting, there is an equilibrium between the solid and the liquid,
that is, the solid is becoming liquid, while the liquid is becoming
solid, in different parts of the mixture.
However, you can take advantage of the fact that during a phase
change of a pure substance, the temperature does not change. For
example, ice-water will remain at 0degC while the phase change (ice
to water) is happening, and will remain at 0degC until all the ice
has melted. Only then will the temperature increase. This is because
any input of heat is being used to increase the potential of the
molecules (solid to liquid) rather than increase the kinetic energy
(and result in a change in temperature) of the molecules.
Be careful though. The previous paragraph describes what happens in
a phase change for a *pure* substance. Ice cream is definitely a
mixture, a solution. It will not behave the same way as a pure
substance. The problem here is that as a solution melts, some
components of the mixture may preferentially melt first, and this
may change the solid's composition. As it does, the melting point
gradually changes. As a result, you will see a slight change in
temperature as the melting occurs (unlike in pure substances where
there is no change).
However, it may still be possible to estimate the melting point of
the ice cream by noting the changes in a time-temperature curve.
I suggest you try an experiment using the purest water you can find
- just for the experience. Make some ice from this water. Immerse
the ice in some pure liquid water, and take the temperature of the
water every minute or so (a TI-temp probe would help here as it can
do this automatically). By drawing the best straight line through
(a) the primarily solid [below 0degC], (b) the melting region, and
(c) the liquid water - you will see that there are three different
slopes. You can then define the melting point as the intersection of
the best straight lines of b and c.
You can then use this experience to do the same for different fat
content ice cream.
Greg (Roberto Gregorius)
Canisius College
Annaliese,
Well, I commend you on (unintentionally?) picking a tremendously
difficult challenge. Ice cream is a very complex substance (believe it
or not). And, it is extremely difficult to measure a rate of melting
for even pure substances, let alone with a viscous liquid like melted
ice cream. When ice cream melts, the outer surface melts first,
leaving a still-frozen core. If you try to observe the change in shape
of a scoop, you will be measuring the rate of flow of the liquid, not
the rate of melting of the ice cream. In effect, you are not measuring
an intrinsic property (look up "intrinsic property" if you do not know
this term) of the ice cream, but rather just measuring the effects of
experimental conditions. This is not a good experiment! Adding more
complexity, ice cream is actually a suspension of water crystals and
air bubbles surrounded by fat globules. So, it is not really 'melting'
the way an ice cube would melt -- you are actually measuring the
increase in mobility of the fat molecules, the melting of the ice
crystals, and the release of the air bubbles from the matrix.
So let us try to avoid these problems rather that trying to solve them.
Rather than trying to measure a "rate of melting", I suggest you try
to design an experiment that is based on measuring an amount of time
(e.g. an end point, not a rate). For example, you could use an apple
corer to take a core of ice cream from a container, and then measure
the amount of time for the height of the core to drop (melt) to a
specified height. Say, a 4" core to melt to a 1" height. You could
perform the experiment in a refrigerator, at room temp, and in an oven
as well to explore the role of temperature on melting and combined
effects of temperature and fat content on melting. The slower the melt
and the faster the flow of the liquid, the more accurate this
experiment will be.
One more caution -- ice cream is extremely fragile -- if it is old or
has been mishandled, the crystalline structure can change significantly -- so
much so that it will impact your experiments. For example, if you melt
and then re-freeze ice cream, the ice crystal size will be different
than freshly (properly) frozen ice cream. One way to design around
this is to intentionally melt and then slowly re-freeze the ice cream
(this will result in larger ice crystals and larger fat globules --
resulting in a less smooth feeling ice cream), and then perform your
melting experiments again. It will be interesting to see which melts
faster.
Last, I suggest you keep thinking of other variables to test in your
experiments -- a good science fair project must have proper
experimental controls. Make sure you are actually testing the intended
cause-and-effect, and not some uncontrolled or unknown variable that is
confounded with the intended variables.
Hope this helps,
Burr Zimmerman
Your suspicion that testing whether the ice cream is/is not melted.
It is a complicated mixture and may not have a "sharp" melting point. As you
know, ice cream softens rather than having a well-defined melting point.
There is another issue you touch on that is a common mistake made by both
students and teachers. It is this: When you frame the question in terms of
"how fast" or "melting speed" you add a lot of variables that are difficult
to track. For example, the "melting speed" is going to depend upon the
temperature difference between the sample and its surroundings. In addition
to being difficult to measure, that temperature difference is really not
what you are interested in. It is just a characterization of the
experimental setup that can have a significant "lag".
Vince Calder
You have fallen victim to a common error made by both students and
teachers, and that is talking about trying to measure the "SPEED" of a
process. We talk about the speed, but when it comes down to actually
measuring the "SPEED" of a process things get very complicated. A lot of
new factors have to be controlled.
You have pointed out just one of the problems, "How do you test if the ice
cream was melted." And you arrived at the crux of the problem, "How do you
measure if the ice cream has melted?" In the case of ice cream this is a
particularly difficult problem. Are you going to measure whether it is the
ice crystals that are the melting point, or is it the softening of the
cream? Or is it something in between. The melting temperature of a complex
mixture such as ice cream is difficult to define.
Vince Calder
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Update: June 2012
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