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Name:Donna
Status: educator
Grade: 9-12
Location: IN
Country: USA
Date: September 2007


Question:
I had my students take time-temperature data for ice water and shortening. The ice water behaved as expected, staying cold until all the ice melted and then temperature increased linearly with time. The shortening temp (starting around 5 C) increased linearly, say 1 degree per minute. Then when melting was happening it increased much faster -- 4-6 degrees a minute, then slowed down again to around 2 or 3 degrees a minute. I was expecting to show that temperature is constant during a phase change, but I am getting the opposite results. The same thing happened with butter, by the way. Can you explain what is happening here?



Replies:
Donna,

Thank you for doing this experiment with your students - it emphasizes the need in science to measure in order to verify our expectations.

I am guessing that in all three cases you had frozen ice cubes, shortening and butter in water and were measuring the temperature change in the water. As such, in the case of the ice cubes, you were only measuring the temperature effects of a phase change. However with the shortening or butter, not only were you seeing the effect of a phase change, but you were also measuring the heat of solvation as the some of the substances in the butter or shortening (which are complex mixtures of different substances) dissolved into the water. Essentially, it appears that as the shortening or butter melted (which, if it were a pure substance wouldn't have shown a temperature change through the phase change), it was also dissolving and releasing heat - which did cause a change in temperature.

Moreover, the change in temperature rise that you noticed after the shortening or butter had completely melted was due to a change in the heat capacity of the solution now present. Since, initially, the rate of temperature change was measured in water, the time-temperature measurements were a function of the specific heat of the water. In the case of ice, you were still measuring this same specific heat. However, in the case of the shortening or butter, you were now measuring the change in temperature over time of the shortening or butter solution in water which has a different specific heat.

You could do this same measurements by freezing the thermometer in the ice, shortening or butter. Unfortunately, the issue of proper contact with the thermometer bulb becomes a problem. Moreover, since butter is a mixture of substances (including water) you might still measure the heat of solvation - although I would suspect, less so since the substances in the butter are already in intimate contact with one another. This might be especially true for the shortening.

Greg (Roberto Gregorius)


Donna-

Two things happens with fats and greases-

1) It is not a single substance. There are multiple distinct kinds of molecules, and so a range of melting points.

2) Even if all melted at the same temp, the latent heat of fusion is much smaller than for water, so the plateau effect is less well enforced.

Together these have so much effect that a grease is often considered to not have a freezing point. It is a thermodynamically distinct type of material that merely gets more viscous with cold. Unlike water, it does not have a sharp discontinuity in the energy-vs-temperature curve. Glasses and some plastics do this too. Even a number of pure substances (not mixtures) do this. By the time they are cold enough to crystallize and release latent heat, they are so viscous that the molecules cannot rearrange in short order, so one cannot ever see a freezing plateau in the lab.

If the experiment design called for trying both water and grease, I think that showing this distinction was the intent.

Another thing might be relevant:

3) Grease is viscous or even semi-solid, not moving while it warms up. In a water/ice mix, all the water is "water-thin", and may move around transporting heat, making the warmest parts of the liquid occasionally touch the coldest parts of the ice, making them more nearly uniform in temperature. You or your students may well have been stirring the ice-water idly. Grease, on the other hand, it is tough to imagine stirring effectively; you would have to really beat it, briskly and continuously, and then you would probably be making the heat that warms it up...

So a single gelatinous mass usually seems to be gradually penetrated by diffusion of heat, and temperature is rarely uniform over a volume.

Afraid I cannot entirely interpret the changing slopes you saw. Each slow place in the warming curve might be due to a family of similar substances in the mix, gradually releasing their latent heat as temperature passes through their range.

Or it might be an artifact of your heat addition rate not being steady. A family might be defined as the same molecular structure except for slightly differing numbers of carbons in some hydrocarbon chain, or the same number of carbons but with branching. If your grease had two slow ranges, that implies it has two relatively distinct chemical families in it. Not sure what they would be.

Jim Swenson



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