lwright
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Joined: Wed Nov 14, 2007 1:26 pm

freezing milk vs water

Postby lwright » Wed Nov 14, 2007 1:40 pm

I helped my son with his science project. He wanted to know which would freeze faster milk or water. In all 3 trials the milk froze before the water. We used 3 teaspoons of each liquid. The milk froze at a average of 90 minutes and the water was an average of 125 minutes.

I thought the water would freeze first. Any reasons or ideas?

ChrisG
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Postby ChrisG » Wed Nov 14, 2007 2:28 pm

Hi,
Interesting project! There are lots of possibilities of what could cause the results you saw. It's worth considering experimental factors that might have affected your results. For example, were the water and milk at the same starting temperature? Did you measure the temperature of the water and milk at the start and end of the experiment? Were the water and milk stored in the same type of container?

If you discover any problems, you can modify and repeat the experiment. If you can satisfy yourself that experimental design was not a factor, then you'll want to consider the physics of the problem. For starters, I would recommend searching on the web for information about thermal conductivity, heat capacity, and latent heat of freezing. These values are available for milk and water. You may need to simplify or dig deeper, depending on the grade level of your child.

I hope that helps,
Chris

sunbeam
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Re: freezing milk vs water

Postby sunbeam » Wed Nov 14, 2007 2:31 pm

lwright wrote:I helped my son with his science project. He wanted to know which would freeze faster milk or water. In all 3 trials the milk froze before the water. We used 3 teaspoons of each liquid. The milk froze at a average of 90 minutes and the water was an average of 125 minutes.

I thought the water would freeze first. Any reasons or ideas?


Hello!

My guess is that the milk started out colder than the water.

The freezing temperature for milk should be lower than that for water. But I'm not sure how that translates to the time it takes for the solution to cool. Perhaps one of the physics or chemistry folks here can answer that.

A more accurate approach will be to have a thermometer in the milk/water. But, it will have to be a precise thermometer, because they will probably freeze at nearly the same temperature. I just looked online and saw that whole milk should freeze at -0.54 degrees C, which is quite close to 0 degrees C, the freezing point for water.

here's the site I found:
http://van.physics.uiuc.edu/qa/listing.php?id=1606

Finally, it would be wonderful if you would encourage your son to post his questions here. This forum is here to help the students learn how to ask questions and do research.

cheers
Laura

Craig_Bridge
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Postby Craig_Bridge » Wed Nov 14, 2007 5:50 pm

From a thermodynamics standpoint, there are two amounts of heat that have to be removed in order to go from a liquid to a solid. One is the amount of heat required to decrease the liquid from its starting temperature to its freezing point and the second is the heat of fusion (the heat required to go from a solid to a liquid).

I did a quick search and didn't come up with the heat of fusion for milk (which would vary by fat content).

The other issue is what salts are in the "water". Soft water will have NaCl or KCl depending on what salt is used in the softner. Hard water will have various mineral content (other salts). These will affect both the freezing point and the heat of fusion.

Unless you use distilled water, then figuring out the heat of fusion and freezing point of the water will require measurements.

The thermal conductivity differences between milk and water might be a small factor; however, given the large differences, I'm guessing the following are more important:
1) Starting temperature differences
2) Differences in the weight of the samples
3) Differences in container shapes, volumes, surface areas
4) The freezing point for your water samples (unless it was distilled water)
5) The heat of fusion difference between the milk and water
-Craig

Craig_Bridge
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Postby Craig_Bridge » Thu Nov 15, 2007 5:20 am

You probably want to look up "homogenization" or "homogenized milk". Among other things Milk can be classified as a suspension or mixture in physical chemistry terms. In other words, there are solids suspended in the liquid.

What this means is that some portion of the milk is already a solid and that portion does not undergo a phase change (liquid to solid) so the heat of fusion does not apply that portion which is already in the solid phase.

If you have access to a centrifuge, you could separate the suspension and figure out a weight ratio of solute to solids. I suspect that if you took the solute and ran a fair freeze race with distilled water your freeze times would be a lot closer. I'm not sure how you could run a fair freeze race as the density of the milk solute and water are different. The more the sample weighs, the more latent heat that must be removed. The smaller the volume, the faster the heat transfer occurs so you have two opposite factors and figuring it out isn't simple.

If you let the milk thaw after freezing, you might also see some separation. Some of the milk solids might not resuspend on its own.

If your student is in a lower grade, you might try experimenting using muddy water or various ice / water mixtures (ones with with different ratios of ice to water).
-Craig


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