The Myth of the Boiling Point
Hasok Chang
Department of History and Philosophy of Science
University of Cambridge
18 October 2007
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Introduction
We all learn at school that pure water always boils at 100°C (212°F), under normal atmospheric pressure. Like surprisingly many things that "everybody knows", this is a myth. We ought to stop perpetuating this myth in schools and universities and in everyday life: not only is it incorrect, but it also conveys misleading ideas about the nature of scientific knowledge. And unlike some other myths, it does not serve sufficiently useful functions.
There are actually all sorts of variations in the boiling temperature of water. For example, there are differences of several degrees depending on the material of the container in which the boiling takes place. And removing dissolved air from water can easily raise its boiling temperature by about 10 degrees centigrade.
The fickleness of the boiling point is something that was once widely known among scientists. It is quite easy to verify, as I have learned in the simple experiments that I show in this paper. And it is still known by some of today's experts. So actually the strange thing is: why don't we all hear about it? Not only that, but why do most of us believe the opposite of what is the case, and maintain it with such confidence? How has a clear falsehood become scientific common sense?
I first became aware of this whole issue a few years ago, in the course of working on my recent book Inventing Temperature (New York: Oxford University Press, 2004), a historical and philosophical treatise on thermometers and the temperature concept (read the Introduction and table of contents of the book).
 The old thermometer whose photo I have put on the cover of the book speaks volumes (click on the picture for a larger version). This instrument, dating from the 1750s, is preserved at the Science Museum in London; the glass stems have broken off, so all we have is the frame, which shows four different scales on it. The third one is the familiar Fahrenheit scale. (The second one, due to Delisle, is "upside down", with 0° at the boiling point and increasing numbers as it gets colder; read more about such scales on pp.160-162 in Inventing Temperature.)
There are two boiling points marked on this thermometer. At the familiar 212°F it says "water boyles vehemently". Down at about 204°F it says "begins to boyle". What is going on here?
You may think that the artisan who made this thermometer must have been pretty incompetent on scientific matters. But it turns out that this thermometer was the work of George Adams, official scientific instrument-maker to King George III. And the idea of two boiling points actually came straight from Isaac Newton, whose temperature scale published in 1701 was indeed the first of Adams's four scales.
History
Stimulated by these oddities from the 18th century, I looked more deeply into the history, to see what people really knew and thought about the boiling point in those early days.  In fact there was so much uncertainty about it that in 1776 the Royal Society of London appointed a special committee charged with making definite recommendations about the "fixed points" of thermometers. The Royal Society committee recorded various types of variations in the boiling temperature of water. Henry Cavendish (portrait on the right), who chaired the committee, left us a rather enigmatic statement in one of his unpublished manuscripts: "The excess of the heat of water above the boiling point is influenced by a great variety of circumstances." (Read more about the Royal Society Committee's findings.)
 Another key member of the committee was Jean-André De Luc (left), Genevan geologist, physicist, meteorologist, theologian and businessman. By this time he was living in England, installed in Windsor as "Reader" to Queen Charlotte. Around 1770 De Luc had made extensive investigations into boiling. He reasoned that in an ordinary boiling situation the layer of water touching the heated surface, where the vapour bubbles form, must be much hotter than the rest of the water. He wanted to find out the temperature of that "first layer", which would be the temperature of "true ebullition". So he took a narrow-necked flask and heated it in a bath of oil (rather than on an open flame), trying to bring the whole body of the water to the same temperature by slow heating with minimal loss of heat at the open surface. But when he did this, De Luc found that the water would not boil normally at all. The bubbles were infrequent and very large, sometimes explosive; the temperature was high and unsteady, sometimes reaching up to 103°C.
A further puzzle awaited De Luc. He noticed that the presence of dissolved air in water induced what seemed like premature boiling. He tried to take the air out by various methods, but in the end decided that he needed to shake a sealed bottle of water for a long time, in addition to all else (remember how shaking a bottle off fizzy drink releases bubbles of gas). He reported: "This operation lasted four weeks, during which I hardly ever put down my flask, except to sleep, to do business in town, and to do things that required both hands. I ate, I read, I wrote, I saw my friends, I took my walks, all the while shaking my water." Four mad weeks of shaking had its rewards. De Luc's precious airless water reached 112.2°C before boiling off explosively. (Read more about De Luc's work on boiling.)
 In the course of the 19th century, further study revealed boiling to be an even more complex and unruly phenomenon than De Luc had glimpsed. For example, in the 1810s Joseph-Louis Gay-Lussac in Paris reported that water boiled at 101.2°C in a glass vessel, while it boiled at exactly 100°C in a metallic vessel. This result became fairly well known, but there was no definitive explanation of it available for a long while. In 1842 François Marcet in Geneva extended Gay-Lussac's work and reported that water could reach over 105°C in a glass vessel treated with hot sulphuric acid. This set off a whole line of research in which different researchers competed with each other to attain higher and higher temperatures of pure liquid water under normal atmospheric pressure. This "superheating" race was won, as far as I can ascertain, by a German named Georg Krebs, who achieved an estimated 200°C in 1869. (Read more about the 19th-century work on superheating.)
Experiments
I was very surprised to learn about these things. I thought I had a pretty good scientific education, but I had never once heard of such things while studying physics as an undergraduate student -- or in my subsequent life as a philosopher and historian of science, in which I have been continually in touch with colleagues investigating odd bits, loose ends, and unresolved questions in science all the time.
I put a detailed account of the historical debate in the first chapter of my book mentioned earlier, but I was still left with a problem of incredulity. Were the 18th- and 19th-century scientists right? Or was this an error like the infamous recent case of "cold fusion" or the older case of "N-rays"? I decided there was only one way to find out: see for myself, in the lab.
I was able to put this thought into practice during the summer of 2004 and then again in 2007, thanks to the hospitality of colleagues at the Department of Chemistry at University College London, and a research grant from the Leverhulme Trust. My experiments were simple enough, but they couldn't have been done without generous help from numerous people.
There are six sets of experiments I would like to present, very briefly summarized below. Click on the links at the end of each summary, to get a full description and the video footage of the highlights, and some further notes on other related experiments and observations.
General notes on the experiments and apparatus used.
Experiment 1. The indefiniteness of the boiling point
Almost as soon as I stepped into the lab, I realized that ordinary boiling was a very complicated and quite indefinite phenomenon, as witnessed by the Adams thermometer discussed above.
Read about this experiment, and watch a video clip.
Read further about related experiments and observations.
Experiment 2. Different temperatures in different vessels
After observing the very ordinary boiling of water with some care, the next obvious thing to do was to test Gay-Lussac's claim that the boiling temperature was affected by the material of the vessel in which the boiling took place. In my experiments, differences ranging over 3°C were easily observed -- lowest in metal, higher in glass, and highest in some ceramic vessels.
Read about this experiment, and watch a video clip.
Read further about related experiments and observations.
Experiment 3. Lower temperature in a hydrophobic vessel
These observations led to further experiments. Gay-Lussac’s explanation for the effect of surface quality was that the boiling temperature was higher when water adhered to the surface of the vessel more strongly. When Marcet developed Gay-Lussac’s work further, he noted that a glass beaker covered in a layer of sulphur, which repelled water, showed a boiling temperature of 99.7°C. On hearing this, my chemist-sponsor Andrea Sella suggested that we could try a modern version of the experiment which, sure enough, yielded significantly lower boiling temperatures. (Note for historians: Aren't there worries about not using the same materials used by the historical scientists?)
Read about this experiment, and watch a video clip.
Experiment 4. The action of boiling chips
Gay-Lussac also reported that throwing in metal filings or even powdered glass into water boiling in a glass vessel lowered the temperature, bringing it closer to 100°C. In fact this idea was later developed into the use of “boiling chips” to avoid superheating and facilitate smooth boiling, a common practice in chemistry labs to this day. Boiling chips enable water to boil at considerably lower temperatures.
Read about this experiment, and watch a video clip.
Read further about related experiments and observations.
(Note on safety and equipment: Experiments 1-4 are really very simple, involving minimal materials and skill, though it may not be trivial to get a supply of distilled water if you are not working in an established laboratory. The next two experiments are just a little bit more involved, and they should certainly not be attempted without eye protection, as there will be bursts of very hot water coming out of flasks.)
Experiment 5. Superheated boiling by slow heating
I carried out a long series of experiments in an attempt to replicate De Luc's work published in 1772, to examine what happens when water is boiled using a gentle source of heat while minimizing heat loss at the surface. In this setup the boiling starts out quite normally at around 100°C, but gradually the water begins to "bump" and "puff". The temperature can easily reach 103-104°C.
Read about this experiment, and watch a video clip.
Read further about related experiments and observations.
Experiment 6. Superheating facilitated by de-gassing
Finally, I tried to replicate De Luc's result on taking dissolved air out of the water. But unlike De Luc I wasn't willing to commit to four weeks of shaking, so I found an alternative method. (Find out more about the "de-gassing" process.) When the de-gassed water is heated up gently, the result is often quite remarkable: nothing visible at all happens for a long while, and then suddenly the water explodes. I have observed temperatures reaching 108-109 °C just before the explosion. Has anyone else out there seen this?
Read about this experiment, and watch a video clip.
Read further about related experiments and observations.
Discussion
From my experiments, I hope you can easily see that the strange observations reported by scientists in the 18th and 19th centuries were generally quite correct. This raises a few very interesting questions.
(1) If the boiling point is so indefinite, how can standard thermometers be made? A great irony: the result that water does not always boil at 100°C was established by means of thermometers which were calibrated on the assumption that it does boil at 100°C. This apparent nonsense is actually not as bad as it sounds. Read more.
(2) If the temperature of boiling water is so variable under quite mundane circumstances, how come we don’t notice that, when most of us boil water on a daily basis? It is only because most people still usually boil water in the type of conditions that prevailed in 19th-century Europe. Am I advocating mad cultural relativism? Read more.
(3) Why haven't scientists come up with a theory to explain these strange phenomena? In fact, there is such a theory, and a well-established body of experimental work to go with the theory -- not so much in physics or chemistry, but in engineering. (I first learned this in the midst of my experiments, thanks to Steve Bramwell and Mike Ewing of the UCL Chemistry Department.) Read more.
Conclusion
My investigation has opened up a lot more questions than it has answered. There are things that I have not managed to understand at all yet, for example exactly how and why dissolved air plays such an important role in the facilitation of boiling. (Read more about remaining questions and further work.) But already, my preliminary study has revealed some significant gaps in the common knowledge of boiling in standard physics and chemistry, especially in the way these subjects are taught, even in higher education. These gaps exist not because science is incapable of filling them, but because science needs to set aside many questions and facts in order to allow its focus on the current cutting-edge of research.
History and philosophy of science can serve the function of preserving and developing aspects of scientific knowledge that are lost and neglected in the process of scientific progress. I would have never learned all the good things about boiling that I have presented in this paper, if I had not learned from the historical sources. And I would not have looked into that history if I had not been investigating philosophical questions about how we can know whether our thermometers are reliable.
Using history and philosophy of science to improve our knowledge of nature is a program of research that I call "complementary science" because it supplements current specialist science without disputing its legitimacy (read more about complementary science). I hope that this brief presentation of a concrete question has given you a glimpse of the potential of this research program, which I plan to pursue for many years to come.
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