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[Phys-l] Boiling Point of water - a mystery?

This was forwarded from chem list-serv; I thought we physics folks would be equally interested in it.


1. A query from a science teacher:
I am an 8th grade science teacher, and we recently did an activity in which the students collected temperature data as they heated ice to go through phase changes. As expected, they had plateaus for melting and boiling points, but one group in particular had a mystery that we can't yet solve.

At one lab station, the "boiling point" was much higher than any other station. In each class, the plateau was around 112 degrees C, while most other station recorded boiling points around 101 or 102 C. Our first guess was that the thermometer they were using (a simple glass/alcohol one) was not calibrated properly, but when we tested it at other stations, it was always within 1 degree of the other thermometers. We also wondered whether the beaker we used might have had some chemical contamination that would raise the boiling point substantially, but this seemed unlikely by the fourth class, as I would expect most contaminants that were stuck to the beaker would have dissolved and disappeared down the drain by the fourth class.

The ice that was used was the same for all stations.

One thing we did notice was that the Bunsen burner at that station was particularly powerful -- perhaps hotter than the other Bunsen burners. We were very careful, however, that the thermometer was always several cm above the glass bottom of the beaker, and was always surrounded by water.

So the question is, how is it possible to heat the water beyond it's boiling point? Can anyone offer any plausible explanations? Or perhaps my understanding of boiling point needs to be enhanced?


2. Response from one of the mebers:

Dear David,

I wish I had more time to respond - as I find this to be a very fascinating problem. By problem, what I mean is what we consider to be "textbook" knowledge and how things work in real life.

The following is a link to a series of podcasts.

Scroll down to April 2007 and there you will find the talk by Hasok Chang based on his book "Inventing Temperature". Dr. Chang is a philosopher of science and he has looked at the intriguing problem of how measurements are made. Specifically he looks at how we have arrived at our standardized thermometers. Related to the experiment you have done with your students, he looks at the HUGE problem scientists of the mid to late 1700's had in establishing the boiling point of water. As you have discovered this is not a trivial point. Besides of course issues of atmospheric pressure, the specific boiling point measured depends on whether the water has been degassed and in what container the water in being heated in. These conditions are related to whether or not points of nucleation are available for vapour bubbles to be formed and then escape from the liquid. In certain cases the water can be superheated well above (10 degrees above) the boiling point.

I highly encourage you to listen to the podcast - he actually describes experiments he did (repeats from the 1700's) to show the effects of container on boiling point of water. His book is also well written!

I hope this helps.


~ Hasan Fakhruddin
Instructor of Physics
The Indiana Academy for Science, Mathematics, and Humanities
Ball State University
Muncie, IN 47306