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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 br>

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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