chang, hasok (2007)a when water does not boil at the boiling point

7/27/2019 CHANG, Hasok (2007)a When Water Does Not Boil at the Boiling Point

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When water does not boil at theboiling pointHasok ChangDepartment 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 late18th century, pioneering scientists hadalreadydiscovered great variations in the boiling temperature ofwater underxed pressure. So, whyhavemost of us beentaught that theboilingpoint ofwateris constant?And,if itis not constant, how can it be used as a ‘xed point’ forthe calibration of thermometers? History of science hasthe answers.

The mirage of ‘true ebullition’One of the 18th-century pioneers of boiling was Jean-Andre ´De Luc (1727–1817) – a Swiss meteorologist, physicist,geologist, mountaineer, theologian and businessman(Figure 1 ). De Luc must have seemed like a madman ashe strolled down the streets of Geneva shaking a ask of water with a thermometer sealed into it. He was in the laststages of completing his long-awaited masterpiece, Investi- gations on the Modications of the Atmosphere , which waspublishedin 1772( Figure 2 ).De Luc wasexploitinga kineticeffect that is familiarto anyone whohas made themistake of shaking a can of zzy drink too vigorously. His aim was toextract all the dissolved air from the uid.

This was part of De Luc’s quest for the elusivephenomenon that he called ‘true ebullition’. Most peoplehad thought distilled water was completely pure, butDe Luc pointed out that it contained plenty of dissolvedair and he found that the air facilitated what seemed likepremature boiling. So, he reasoned, the air needed to beremoved:

This operation lasted four weeks, during which Ihardly ever put down my ask, except to sleep, todo business in town, and to do things that requiredboth hands. I ate, I read, I wrote, I saw my friends, Itook my walks, all the while shaking my water [1].

Four mad weeks of shaking had its rewards. De Lucreported that the de-gassed water exhibited very strangebehaviour À it would not boil at all at the normal boiling point; instead, it became ‘superheated’ to $ 112 8 C and thenexploded.

This superheating was consistent with what De Luc hadseen in an earlier series of investigations, in which herealized that boiling, as usually performed, was quite acrude operation. When one puts a pot full of water on anopen ame, the container and the ‘rst’ layer of waterdirectly in contact with the container are hotter than

the rest of thewater. Boiling occurswhen bubbles of vapourform in that rst layer, but the ‘boiling temperature’ istaken with the thermometer placed in the main body of thewater. This was not a coherent experiment. Because it wasimpossible to put a thermometer into the rst, extremely thin layer of water, he sought instead to bring the wholebody of water to the same temperature. De Luc thought hecould achieve this by slow heating with a gentle heat-source, while minimizing heat-loss from the water. To this

end, he employed a round ask with a long, thin neck,which he plunged into a hot bath of nut oil. When he didthis, he encountered a surprising phenomenon, which latercame to be called ‘bumping’. The water in this arrangementoften boiled in an irregular way by producing large,occasional bubbles of vapour; sometimes the bubbles wereexplosive enough to throw some of the water out of theask. During bumping, the temperature of the wateructuated between 100 8 C and somewhere over 103 8 C[2]. And all this was before De Luc even shook the dissolvedair out of the water.

The investigation of boiling took De Luc many monthsand revealed more and more complexities, until he recog-nized six distinct phenomena, all of which might in somesense qualify as ‘boiling’. The 15-chapter supplement onthe variations of the temperature of boiling water, whichDe Luc added to his Investigations , is testimony to thecomplexity of his ndings [3]. At the end of his long searchfor ‘true ebullition’, he ended up not knowing what boiling was at all or at what temperature one could say it hap-pened. He issued the following words of caution about thexed points of thermometers [4]:

Today people believe that they are in securepossession of these points, and pay little attentionto the uncertainties that even the most famous menhad regarding this matter, nor to the kind of anarchy that resulted from such uncertainties, from which westill have not emerged at all.

The Royal Society committee and the steam pointFive years later De Luc was in London, serving on anillustrious seven-man committee appointed by the RoyalSociety to make denitive recommendations regarding thexed points of thermometers. His business had collapsedshortly after the publication of his book, and he immigratedto England, where he was installed in Windsor as ‘Reader’to Queen Charlotte, the consort of George III.

The Royal Society committee, chaired by the enigmaticaristocrat Henry Cavendish (1731–1810), investigatedmany suspected causes of variation in the temperature

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Corresponding author: Chang, H. ( [email protected] ). Available online 2 March 2007.

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of boiling. One of the issues that drew the committee’sattention was the common claim that the temperaturedepended on the ‘degree of boiling’. This idea can be tracedback to Isaac Newton, who recorded, on his own idiosyn-cratic scale of temperature, that water began to boil at 33 8

and boiled vehemently at 34 8 –34.5 8 , which is roughly equivalent to a range of 5 8 –8 8 on the Fahrenheit scale,in which boiling took place [5]. This belief is exhibited

beautifully in a thermometer frame preserved at theScience Museum in London, which shows two boiling points: ‘water boils vehemently’ at 212 8 F, and ‘begins toboil’ at 204 8 F (Figure 3 ). This instrument was the work of George Adams, ofcial ‘Mathematical Instrument Maker’to George III. De Luc also reported similar observations inhis 1772 text [6]; however, somehow this effect was notdetected in any consistent way in the experiments carriedout by the Royal Society committee, and it is not clear whatDe Luc’s personal opinion was about this matter [7].

The committee, however, clearly recognized thephenomenon of superheating. In Cavendish’s words, froman unpublished manuscript probably dating from around

1780: ‘The excess of the heat of water above the boiling point is inuenced by a great variety of circumstances’ [8].Cavendish’s suggestion, which the committee adopted asits ofcial recommendation, was to use the temperature of boiled-off steam, not boiling water [9]. What the publishedreport of the committee does not reveal is that De Luc wasnot convinced that the temperature of steam would bemore constant than that of boiling water [10] . Cavendishargued that even in superheated water, a bubble of steamrising inside it would be at the normal boiling point,because the water right around it would be cooled downto the boiling point by the removal of latent heat asevaporation took place into the bubble. His reasoning was based on the assumption that water directly in contactwith steam or air would always turn into steam as soon asit reached the normal boiling point, whereas water not incontact with steam or air would ‘bear a much greater heatwithout being changed into steam, namely that which MrDe 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 downbelow the boiling point without condensing into liquidwater. Correspondence between the two men from thistime indicates that they could not come to a theoreticalagreement on these issues. ‘Let us then, Sir, proceed withimmediate tests without dwelling on causes,’ De Lucsuggested [12] . Judging from the ofcial report of the

committee, experiments showed that Cavendish was cor-rect in thinking that ‘steam must afford a considerably more exact method of adjusting the boiling point thanwater’ [13] . This recommendation became widely adopted.

Surface effects and the superheating raceSo, by the late 1770s, the question of the boiling pointseemed to have been reasonably resolved. However, thematter was reopened in the 1810s, when the highly regarded French physicist and chemist Joseph-LouisGay-Lussac (1778–1850) reported that water boiled at101.2 8 C in a glass vessel but at 100 8 C in a metallic one[14] . This observation was widely reported and generally

Figure 1 . Portrait of De Luc (Geneva, Bibliotheque Publique et Universitaire,Collections iconographiques).

Figure 2 . Cover page of De Luc (1772) (British Library).

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Aitken made this discovery during his quest to learnwhy the industrial cities of Victorian Britain were blightedby heavy fogs. He suspected that the ne particles of dustoating in the air were aiding the condensation of tiny water droplets, thus generating fog. He demonstrated thecogency of his idea by showing that there was no fogging indust-free air. Today, historians of science mainly focus onhow 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 con-densations in its path, thus generating a visible trajectory).For Aitken himself, the signicant implications werealways in meteorology [19] :

If there was no dust in the air there would be no fogs,no clouds, no mists, and probably no rain . . . Whenthe air got into the condition in which rain falls – thatis, burdened with supersaturated vapour – it wouldconvert everything on the surface of the earth into acondenser, on which it would deposit itself. Every blade of grass and every branch of tree would dripwith moisture deposited by the passing air; ourdresses would become wet and dripping, and umbrel-las useless . . .

It is interesting to note Aitken’s general view regarding changes of state. In opposition to the common idea thatchanges of state simply happened at certain temperature–pressure combinations, Aitken argued that ‘something more than mere temperature’ was required, namely a ‘freesurface’ at which the change could take place. In conden-sation, dust particles provide the necessary free surfaces,and in vapour-formation, a liquid–gas interface serves thatrole. The temperatures at which these changes of statehappen are not xed; they depend on the degree to whichappropriate free surfaces are available. Aitken explainedhow this general account facilitated his major discovery [19] :

I knew that water could be cooled below thefreezing-point without freezing. I was almost certainice could be heated above the freezing-point withoutmelting. I had shown that water could be heatedabove the boiling-point . . . Arrived at this point,the presumption was very strong that water vapourcould be cooled below the boiling-point . . . withoutcondensing.

Lessons for todayThere is a paradox in the history of boiling. The result that

water does not always boil at 1008

C was established by using thermometers that were calibrated on the assump-tion that water always boils at 100 8 C. This apparentnonsense is actually not as bad as it sounds. What scien-tists have been able to do is identify particular situations inwhich the boiling temperature is quite well xed; ther-mometers can be calibrated in those situations, and thenthey 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 stablesystem of thermometry, and develop a theory of thermo-dynamics in which the boiling point is sharply dened. Itmight be said that scientists did not discover the xity of

the boiling point, but they learned how to make it xed,although of course only in a way that was allowed by nature.

But, if the boiling temperature is actually so variableunder mundane everyday circumstances, why do not wenotice it, when most of us boil water on a daily basis? It isbecause most people still boil water in the type of con-ditions that prevailed in 18th-century Europe – in wide-

open vessels with intense heating from the bottom. We canimagine that the ‘standard’ process of boiling could bedifferent in a different sort of civilisation. For example,if we had no access to ames but easy access to hot sand,boiling would have to be done in narrow-necked asksburied in the sand, routinely producing the kind of super-heated bumpy boiling that De Luc observed. Or imagine allheating being done in microwave ovens, which haverecently becoming notorious for producing superheatedwater 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 existtoday in engineering. Modern engineers have been accumu-lating experimental and theoretical knowledgeof the differ-enttypesof boilingthat take place in various situations [20] .In the engineering treatises on boiling, there are detailedexplanations of the difference that the quality of the vesselsurface makes. The engineer’s paradigmatic representationof boiling is the ‘boiling curve’, plotting the rate of heattransferagainst the degree of ‘surface superheat’ [21] . Here,superheating in the bubble-forming layer of water is takenfor granted, and the boiling curve represents the con-sequences of the various degrees of superheating; all thisis not even expressible in the standard physics discourse,which is based on the idealised assumption that superheat-ing never occurs. Also, the main variable of interest in theboiling curve is the rate of heat transfer. In this context, thewater temperature away from the surface layer is of sec-ondary interest; it is freelyadmittedto be quite variable andit is not even represented in the boiling curve.

Clearly, there are some signicant gaps in theknowledge of boiling presented in today’s standard physicsand chemistry textbooks. These exist not because science isincapable of lling them, but because science needs to setaside many questions and facts in order to maintain itsfocus on the current cutting-edge of research. History andphilosophy of science can function as ‘complementary science’, preserving and developing aspects of scientic

knowledge that are lost and neglected in the very processof scientic progress [22] .

References1 De Luc, J.A. (1772) Recherches sur les Modications de l’Atmosphe `re

(Vol. 2), p. 3872 Ibid., pp. 362–3643 Ibid., pp. 227–438; and Chang, H. (2004) Inventing Temperature:

Measurement and Scientic Progress, pp. 15–27, Oxford University Press

4 De Luc, J.A. (1772) Recherches sur les Modications de l’Atmosphe `re(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 Bookin Physics (Magie, W.F., ed.) pp. 125–128, McGraw-Hill

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6 De Luc, J.A. (1772) Recherches sur les Modications de l’Atmosphe `re(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 xedpoints of thermometers; and of the precautions necessary to be used inmakingexperiments with thoseinstruments. Phil. Trans. R. Soc. Lond.67, 816–857 (on 819–820)

8 Cavendish, H. (1921) Theory of boiling. In The Scientic 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. 359–36010 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 Scientic Papers of the HonourableHenry Cavendish, F.R.S. (Vol. 2, Chemical andDynamical )(Thorpe, E., ed.), pp. 354–362 (on p. 354), Cambridge University Press

12 Letter, De Luc to Cavendish (19 February 1777), reprinted inJungnickel, 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 Scientic Papers of the HonourableHenry Cavendish, F.R.S. (Vol. 2, Chemical andDynamical )(Thorpe, E., ed.), pp. 354–362 (on pp. 359–360), Cambridge University Press

14 Biot, J.B. (1816) Traite ´ de Physique Expe ´ rimentale et Mathe ´ matique,pp. 42–43, Deterville

15 Marcet, F. (1842) Recherches sur certaines circonstances qui inuentsur la tempe ´rature du point d’e ´bullition des liquides. Bibliothe `queUniverselle (new series) 38, 388–411

16 Donny, F. (1846) Me ´moire sur la cohe ´sion des liquides, et sur leuradhe´rence aux corps solides. Annales de Chimie et de Physique (3rdseries) 16, 167–190 (on 187–188)

17 Dufour, L. (1861) Recherches sur l’e ´bullition des liquides. Archives des Sciences Physiques et Naturelles (new series) 12, 210–266 (on p. 225)

18 Gernez, D. (1875) Recherches sur l’e ´bullition. 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 acontinuation of science by other means, In Inventing Temperature: Measurement and Scientic Progress , pp. 235–250, Oxford University Press

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chang, hasok (2007)a when water does not boil at the boiling point

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