Review
Endeavour
Vol.31 No.1
When water does not boil at the
boiling point
Hasok Chang
Department of Science and Technology Studies, University College London, Gower Street, London WC1E 6BT, UK
Every schoolchild learns that, under standard pressure,
pure water always boils at 100 -C. Except that it does not.
By the late 18th century, pioneering scientists had already
discovered great variations in the boiling temperature of
water under fixed pressure. So, why have most of us been
taught that the boiling point of water is constant? And, if it
is not constant, how can it be used as a ‘fixed point’ for
the calibration of thermometers? History of science has
the answers.
The mirage of ‘true ebullition’
One of the 18th-century pioneers of boiling was Jean-André
De Luc (1727–1817) – a Swiss meteorologist, physicist,
geologist, mountaineer, theologian and businessman
(Figure 1). De Luc must have seemed like a madman as
he strolled down the streets of Geneva shaking a flask of
water with a thermometer sealed into it. He was in the last
stages of completing his long-awaited masterpiece, Investigations on the Modifications of the Atmosphere, which was
published in 1772 (Figure 2). De Luc was exploiting a kinetic
effect that is familiar to anyone who has made the mistake of
shaking a can of fizzy drink too vigorously. His aim was to
extract all the dissolved air from the fluid.
This was part of De Luc’s quest for the elusive
phenomenon that he called ‘true ebullition’. Most people
had thought distilled water was completely pure, but
De Luc pointed out that it contained plenty of dissolved
air and he found that the air facilitated what seemed like
premature boiling. So, he reasoned, the air needed to be
removed:
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 [1].
Four mad weeks of shaking had its rewards. De Luc
reported that the de-gassed water exhibited very strange
behaviour it would not boil at all at the normal boiling
point; instead, it became ‘superheated’ to 112 8C and then
exploded.
This superheating was consistent with what De Luc had
seen in an earlier series of investigations, in which he
realized that boiling, as usually performed, was quite a
crude operation. When one puts a pot full of water on an
open flame, the container and the ‘first’ layer of water
directly in contact with the container are hotter than
Corresponding author: Chang, H. ([email protected]).
Available online 2 March 2007.
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the rest of the water. Boiling occurs when bubbles of vapour
form in that first layer, but the ‘boiling temperature’ is
taken with the thermometer placed in the main body of the
water. This was not a coherent experiment. Because it was
impossible to put a thermometer into the first, extremely
thin layer of water, he sought instead to bring the whole
body of water to the same temperature. De Luc thought he
could achieve this by slow heating with a gentle heatsource, while minimizing heat-loss from the water. To this
end, he employed a round flask with a long, thin neck,
which he plunged into a hot bath of nut oil. When he did
this, he encountered a surprising phenomenon, which later
came to be called ‘bumping’. The water in this arrangement
often boiled in an irregular way by producing large,
occasional bubbles of vapour; sometimes the bubbles were
explosive enough to throw some of the water out of the
flask. During bumping, the temperature of the water
fluctuated between 100 8C and somewhere over 103 8C
[2]. And all this was before De Luc even shook the dissolved
air out of the water.
The investigation of boiling took De Luc many months
and revealed more and more complexities, until he recognized six distinct phenomena, all of which might in some
sense qualify as ‘boiling’. The 15-chapter supplement on
the variations of the temperature of boiling water, which
De Luc added to his Investigations, is testimony to the
complexity of his findings [3]. At the end of his long search
for ‘true ebullition’, he ended up not knowing what boiling
was at all or at what temperature one could say it happened. He issued the following words of caution about the
fixed points of thermometers [4]:
Today people believe that they are in secure
possession of these points, and pay little attention
to the uncertainties that even the most famous men
had regarding this matter, nor to the kind of anarchy
that resulted from such uncertainties, from which we
still have not emerged at all.
The Royal Society committee and the steam point
Five years later De Luc was in London, serving on an
illustrious seven-man committee appointed by the Royal
Society to make definitive recommendations regarding the
fixed points of thermometers. His business had collapsed
shortly after the publication of his book, and he immigrated
to England, where he was installed in Windsor as ‘Reader’
to Queen Charlotte, the consort of George III.
The Royal Society committee, chaired by the enigmatic
aristocrat Henry Cavendish (1731–1810), investigated
many suspected causes of variation in the temperature
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Endeavour Vol.31 No.1
Figure 1. Portrait of De Luc (Geneva, Bibliothèque Publique et Universitaire,
Collections iconographiques).
Figure 2. Cover page of De Luc (1772) (British Library).
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of boiling. One of the issues that drew the committee’s
attention was the common claim that the temperature
depended on the ‘degree of boiling’. This idea can be traced
back to Isaac Newton, who recorded, on his own idiosyncratic scale of temperature, that water began to boil at 338
and boiled vehemently at 348–34.58, which is roughly
equivalent to a range of 58–88 on the Fahrenheit scale,
in which boiling took place [5]. This belief is exhibited
beautifully in a thermometer frame preserved at the
Science Museum in London, which shows two boiling
points: ‘water boils vehemently’ at 212 8F, and ‘begins to
boil’ at 204 8F (Figure 3). This instrument was the work of
George Adams, official ‘Mathematical Instrument Maker’
to George III. De Luc also reported similar observations in
his 1772 text [6]; however, somehow this effect was not
detected in any consistent way in the experiments carried
out by the Royal Society committee, and it is not clear what
De Luc’s personal opinion was about this matter [7].
The committee, however, clearly recognized the
phenomenon of superheating. In Cavendish’s words, from
an unpublished manuscript probably dating from around
1780: ‘The excess of the heat of water above the boiling
point is influenced by a great variety of circumstances’ [8].
Cavendish’s suggestion, which the committee adopted as
its official recommendation, was to use the temperature of
boiled-off steam, not boiling water [9]. What the published
report of the committee does not reveal is that De Luc was
not convinced that the temperature of steam would be
more constant than that of boiling water [10]. Cavendish
argued that even in superheated water, a bubble of steam
rising inside it would be at the normal boiling point,
because the water right around it would be cooled down
to the boiling point by the removal of latent heat as
evaporation took place into the bubble. His reasoning
was based on the assumption that water directly in contact
with steam or air would always turn into steam as soon as
it reached the normal boiling point, whereas water not in
contact with steam or air would ‘bear a much greater heat
without being changed into steam, namely that which Mr
De Luc calls the heat of ebullition’ [11].
De Luc was not so sure. He was also not persuaded by
Cavendish’s argument that steam could not cool down
below the boiling point without condensing into liquid
water. Correspondence between the two men from this
time indicates that they could not come to a theoretical
agreement on these issues. ‘Let us then, Sir, proceed with
immediate tests without dwelling on causes,’ De Luc
suggested [12]. Judging from the official report of the
committee, experiments showed that Cavendish was correct in thinking that ‘steam must afford a considerably
more exact method of adjusting the boiling point than
water’ [13]. This recommendation became widely adopted.
Surface effects and the superheating race
So, by the late 1770s, the question of the boiling point
seemed to have been reasonably resolved. However, the
matter was reopened in the 1810s, when the highly
regarded French physicist and chemist Joseph-Louis
Gay-Lussac (1778–1850) reported that water boiled at
101.2 8C in a glass vessel but at 100 8C in a metallic one
[14]. This observation was widely reported and generally
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Endeavour
Vol.31 No.1
9
Figure 3. Photograph of the Adams thermometer (Science Museum/Science and Society Picture Library; inventory no. 1927–1745).
accepted (Box 1). The next major step in investigating the
effect of the vessel came from the work of François Marcet
(1803–1883) in Geneva in the 1840s [15]. Marcet produced
superheating beyond 105 8C in ordinary (not de-gassed)
water, by boiling it in a clean glass vessel in which strong
sulphuric acid had been heated; apparently, the acid modified the surface of the glass so that the production of vapour
became more difficult. Gay-Lussac and Marcet both theorized that the effect had to do with varying degrees of
adhesion between water and different solid surfaces. To
confirm this idea, Marcet coated a glass vessel with sulphur, which was thought to repel water, and produced
boiling at a temperature of 99.7 8C. Thankfully for thermometry, he found that the temperature of the steam emerging from water boiling at various temperatures was fairly
constant, for example, being only a few tenths of a degree
over 100 8C even when the water temperature was over
105 8C.
Box 1. Was De Luc deluded?
Still incredulous about the reported variations of the boiling
temperature, I endeavoured to see if De Luc’s and Gay-Lussac’s
experiments could be replicated. I was surprised and delighted to
see that most of their key results were readily reproducible.
(1) Boiling is an indefinite and variegated phenomenon, which can
easily be seen to take place over a range of temperatures.
(2) Surface quality makes a significant difference in the types and
temperatures of boiling.
(3) De-gassing causes serious bumping and superheating.
For further details, see reports and video footage of experiments on
http://www.ucl.ac.uk/sts/chang. This experimental work was made
possible by the generosity of the Chemistry Department at UCL,
especially Andrea Sella and Crosby Medley, and funds from the
Leverhulme/ESRC project grant on the nature of evidence.
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Superheating became a clearly recognized object of
study after Marcet’s work, stimulating a string of virtuoso experimental performances vying for record temperatures. François Donny, a chemist at the University
of Ghent, combined Marcet’s insight on adhesion with a
revival of De Luc’s idea about the role of dissolved air,
and produced a stunning 137 8C using airless water in
his own special instrument. Donny declared: ‘The faculty
to produce ordinary ebullition cannot actually be considered an inherent property of liquids, because they
show it only when they contain a gaseous substance in
solution, which is to say only when they are not in a state
of purity’ [16].
In 1861, the work of Louis Dufour, Professor of Physics
at the Academy of Lausanne, added yet another major
factor for consideration. Dufour argued that contact with a
solid surface was the crucial factor in facilitating ebullition,
and demonstrated the soundness of his idea by bringing
drops of water floating in other liquids up to 178 8C, without even de-gassing [17]. In 1869 even Dufour was outdone
when Georg Krebs achieved an estimated 200 8C with an
improvement of Donny’s technique [18].
Dust, fog and the steam point
Even such extreme superheating did not disturb
thermometry because of the steadiness of the steam point.
However, the physics of steam also turned out to be far
from straightforward. A century after the Royal Society
committee, De Luc’s doubts about the fixity of the steam
point came back to haunt physics when the Scottish
meteorologist John Aitken (1839–1919) showed that steam
could become ‘supersaturated’, that is, cooled below the
boiling point without condensing.
Review
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Endeavour Vol.31 No.1
Aitken made this discovery during his quest to learn
why the industrial cities of Victorian Britain were blighted
by heavy fogs. He suspected that the fine particles of dust
floating in the air were aiding the condensation of tiny
water droplets, thus generating fog. He demonstrated the
cogency of his idea by showing that there was no fogging in
dust-free air. Today, historians of science mainly focus on
how Aitken’s work led to Charles Wilson’s invention of the
cloud chamber (in which a charged particle passing
through supersaturated vapour triggers a series of condensations in its path, thus generating a visible trajectory).
For Aitken himself, the significant implications were
always in meteorology [19]:
If there was no dust in the air there would be no fogs,
no clouds, no mists, and probably no rain . . . When
the air got into the condition in which rain falls – that
is, burdened with supersaturated vapour – it would
convert everything on the surface of the earth into a
condenser, on which it would deposit itself. Every
blade of grass and every branch of tree would drip
with moisture deposited by the passing air; our
dresses would become wet and dripping, and umbrellas useless . . .
It is interesting to note Aitken’s general view regarding
changes of state. In opposition to the common idea that
changes of state simply happened at certain temperature–
pressure combinations, Aitken argued that ‘something
more than mere temperature’ was required, namely a ‘free
surface’ at which the change could take place. In condensation, dust particles provide the necessary free surfaces,
and in vapour-formation, a liquid–gas interface serves that
role. The temperatures at which these changes of state
happen are not fixed; they depend on the degree to which
appropriate free surfaces are available. Aitken explained
how this general account facilitated his major discovery
[19]:
I knew that water could be cooled below the
freezing-point without freezing. I was almost certain
ice could be heated above the freezing-point without
melting. I had shown that water could be heated
above the boiling-point . . . Arrived at this point,
the presumption was very strong that water vapour
could be cooled below the boiling-point . . . without
condensing.
Lessons for today
There is a paradox in the history of boiling. The result that
water does not always boil at 100 8C was established by
using thermometers that were calibrated on the assumption that water always boils at 100 8C. This apparent
nonsense is actually not as bad as it sounds. What scientists have been able to do is identify particular situations in
which the boiling temperature is quite well fixed; thermometers can be calibrated in those situations, and then
they can be used to investigate the variations of the boiling
temperature in other situations. That is how 19th-century
physicists and chemists were able to maintain a stable
system of thermometry, and develop a theory of thermodynamics in which the boiling point is sharply defined. It
might be said that scientists did not discover the fixity of
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the boiling point, but they learned how to make it fixed,
although of course only in a way that was allowed by
nature.
But, if the boiling temperature is actually so variable
under mundane everyday circumstances, why do not we
notice it, when most of us boil water on a daily basis? It is
because most people still boil water in the type of conditions that prevailed in 18th-century Europe – in wideopen vessels with intense heating from the bottom. We can
imagine that the ‘standard’ process of boiling could be
different in a different sort of civilisation. For example,
if we had no access to flames but easy access to hot sand,
boiling would have to be done in narrow-necked flasks
buried in the sand, routinely producing the kind of superheated bumpy boiling that De Luc observed. Or imagine all
heating being done in microwave ovens, which have
recently becoming notorious for producing superheated
water that boils over violently when instant coffee is added.
We can also imagine that a different theory of boiling
might have developed if people had been dealing primarily
with non-standard situations. In fact, it is not necessary to
speculate. A different kind of theory of boiling does exist
today in engineering. Modern engineers have been accumulating experimental and theoretical knowledge of the different types of boiling that take place in various situations [20].
In the engineering treatises on boiling, there are detailed
explanations of the difference that the quality of the vessel
surface makes. The engineer’s paradigmatic representation
of boiling is the ‘boiling curve’, plotting the rate of heat
transfer against the degree of ‘surface superheat’ [21]. Here,
superheating in the bubble-forming layer of water is taken
for granted, and the boiling curve represents the consequences of the various degrees of superheating; all this
is not even expressible in the standard physics discourse,
which is based on the idealised assumption that superheating never occurs. Also, the main variable of interest in the
boiling curve is the rate of heat transfer. In this context, the
water temperature away from the surface layer is of secondary interest; it is freely admitted to be quite variable and
it is not even represented in the boiling curve.
Clearly, there are some significant gaps in the
knowledge of boiling presented in today’s standard physics
and chemistry textbooks. These exist not because science is
incapable of filling them, but because science needs to set
aside many questions and facts in order to maintain its
focus on the current cutting-edge of research. History and
philosophy of science can function as ‘complementary
science’, preserving and developing aspects of scientific
knowledge that are lost and neglected in the very process
of scientific progress [22].
References
1 De Luc, J.A. (1772) Recherches sur les Modifications de l’Atmosphère
(Vol. 2), p. 387
2 Ibid., pp. 362–364
3 Ibid., pp. 227–438; and Chang, H. (2004) Inventing Temperature:
Measurement and Scientific Progress, pp. 15–27, Oxford University
Press
4 De Luc, J.A. (1772) Recherches sur les Modifications de l’Atmosphère
(Vol. 1), p. 331
5 Newton, I. (1701) Scala graduum caloris. Calorum descriptiones &
signa. Phil. Trans. R. Soc. Lond. 22, 824–829; and (1935) A Source Book
in Physics (Magie, W.F., ed.) pp. 125–128, McGraw-Hill
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Endeavour
Vol.31 No.1
6 De Luc, J.A. (1772) Recherches sur les Modifications de l’Atmosphère
(Vol. 1), pp. 351–352
7 Cavendish, H. et al. (1777) The report of the committee appointed by
the Royal Society to consider of the best method of adjusting the fixed
points of thermometers; and of the precautions necessary to be used in
making experiments with those instruments. Phil. Trans. R. Soc. Lond.
67, 816–857 (on 819–820)
8 Cavendish, H. (1921) Theory of boiling. In The Scientific Papers of the
Honourable Henry Cavendish, F.R.S. (Vol. 2, Chemical and Dynamical)
(Thorpe, E., ed.), pp. 354–362 (on p. 359), Cambridge University Press
9 Ibid., pp. 359–360
10 Letter, De Luc to Cavendish (19 February 1777), reprinted in
Jungnickel, C. and McCormmach, R. (1999) Cavendish: The
Experimental Life, pp. 546–551, Bucknell University Press
11 Cavendish, H. (1921) Theory of Boiling. In The Scientific Papers of the
Honourable Henry Cavendish, F.R.S. (Vol. 2, Chemical and Dynamical)
(Thorpe, E., ed.), pp. 354–362 (on p. 354), Cambridge University Press
12 Letter, De Luc to Cavendish (19 February 1777), reprinted in
Jungnickel, C. and McCormmach, R. (1999) Cavendish: The
Experimental Life, p. 547 and p. 550, Bucknell University Press
13 Cavendish, H. (1921) Theory of boiling. In: The Scientific Papers of the
Honourable Henry Cavendish, F.R.S. (Vol. 2, Chemical and Dynamical)
(Thorpe, E., ed.), pp. 354–362 (on pp. 359–360), Cambridge University
Press
11
14 Biot, J.B. (1816) Traité de Physique Expérimentale et Mathématique,
pp. 42–43, Deterville
15 Marcet, F. (1842) Recherches sur certaines circonstances qui influent
sur la température du point d’ébullition des liquides. Bibliothèque
Universelle (new series) 38, 388–411
16 Donny, F. (1846) Mémoire sur la cohésion des liquides, et sur leur
adhérence aux corps solides. Annales de Chimie et de Physique (3rd
series) 16, 167–190 (on 187–188)
17 Dufour, L. (1861) Recherches sur l’ébullition des liquides. Archives des
Sciences Physiques et Naturelles (new series) 12, 210–266 (on p. 225)
18 Gernez, D. (1875) Recherches sur l’ébullition. Annales de Chimie et de
Physique (5th series) 4, 335–401 (on 354)
19 Aitken, J. (1880–1881). On dust, fogs, and clouds. Trans. R. Soc. Edn.
30 (1), 337–368 (on 341–342)
20 Hewitt, G.F. et al., eds (1997) International Encyclopedia of Heat and
Mass Transfer, CRC Press
21 Incropera, F.P. and DeWitt, D.P. (1996) Fundamentals of Heat and
Mass Transfer, (4th edn), p. 540, Wiley
22 For an exposition of this mode of work in history and philosophy of
science, which I call ‘complementary science’, see Chang, H. (2004)
Complementary science – history and philosophy of science as a
continuation of science by other means, In Inventing Temperature:
Measurement and Scientific Progress, pp. 235–250, Oxford University
Press
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