A lesson in the physics laboratory on the superheating of waterConcetto Gianino

Citation: American Journal of Physics 75, 496 (2007); doi: 10.1119/1.2719201 View online: http://dx.doi.org/10.1119/1.2719201 View Table of Contents: http://scitation.aip.org/content/aapt/journal/ajp/75/6?ver=pdfcov Published by the American Association of Physics Teachers Articles you may be interested in Superheating of liquid xenon in metal tubes J. Chem. Phys. 131, 064708 (2009); 10.1063/1.3203208 New technique for visualizing microboiling phenomena and its application to water pulse heated by a thin metalfilm Rev. Sci. Instrum. 77, 063706 (2006); 10.1063/1.2206560 Direct observation of a liquid film under a vapor environment in a pool boiling using a nanofluid Appl. Phys. Lett. 86, 134107 (2005); 10.1063/1.1873053 Bubble Behavior in Subcooled Pool Boiling of Water under Reduced Gravity AIP Conf. Proc. 654, 142 (2003); 10.1063/1.1541288 Thermal and dynamic evolution of a spherical bubble moving steadily in a superheated or subcooled liquid Phys. Fluids 10, 1256 (1998); 10.1063/1.869654

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A lesson in the physics laboratory on the superheating of waterConcetto GianinoHigh School “Q. Cataudella,” Scicli (RG), Italy

�Received 2 October 2006; accepted 9 February 2007�

The theory of water vaporization is used to understand the phenomenon of boiling and themechanisms of bubble formation. A simple experiment was done to verify the theory and measurethe bubble radius under superheating conditions. Experimental results are in good agreement withcalculated values. © 2007 American Association of Physics Teachers.

�DOI: 10.1119/1.2719201�

I. EVAPORATION AND BOILING

Vaporization, or the passage of water from a liquid to avapor, can happen in two ways: by evaporation or byboiling.1 Evaporation occurs at any temperature and consistsin the escape to air of the most energetic molecules from thesurface of the water. Boiling is accompanied by the forma-tion of bubbles. These bubbles, which are full of saturatedvapor, go up to the surface and then free the vapor that theycontain to the air.1,2 Unlike evaporation, boiling involves theentire liquid and takes place at a single temperature calledthe boiling point. The boiling point depends on the atmo-spheric pressure and on the concentration of the substancesdissolved in the water. For instance, for distilled water at thenormal atmospheric pressure at sea level �101 325 Pa�, theboiling point is 100 °C.3,4

II. BOILING AND THE PRESSURE OF THESATURATED VAPOR

We now discuss how and why water boils. Many small,almost invisible, air bubbles are dissolved in water and arealso absorbed in the microscopic interstices of the walls ofthe container.3 These air bubbles are saturated with vapor,which exerts a pressure on the surface of the air-water inter-face of the bubble, which is the pressure of the saturatedvapor.1,2

The pressure of the saturated vapor increases with thetemperature.4 To begin boiling, the pressure of the saturatedvapor has to be slightly higher than that of the external at-mospheric pressure. For a bubble inside a liquid to expand, itis necessary that the inside pressure of the saturated vapor pinequal the sum of the atmospheric pressure p0, the hydrostaticpressure ph, and the pressure due to the surface bubble’scurvature pC:1

pin = p0 + ph + pC. �1�

The hydrostatic pressure is equal to �gh, where � is thewater density, g is the gravitational acceleration, and h is thedepth of the bubble of air with respect to the water surfacelevel in the container. The curvature pressure equals 2� /r,1

where � is the surface tension of water and r is the bubbleradius. Thus, Eq. �1� can be written as

pin = p0 + �gh +2�

r. �2�

The hydrostatic pressure does not have an important role

in determining the boiling point because for depths of the

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order of centimeters the hydrostatic pressure is of order of afew kPa, which is negligible compared to atmospheric pres-sure.

The pressure due to the surface bubble’s curvature pC de-pends on the dimensions of the bubble. For example, forbubbles of micrometer dimensions, with ��7�10−3 N/m,5

the curvature pressure is the order of

pC �2 � 7 � 10−2

10−6 � 105 Pa. �3�

However, in water there are usually larger bubbles, of theorder of tenths of millimeters, for which pC is negligiblecompared to the external atmospheric pressure.

We conclude that under most circumstances, boiling be-gins when pin� p0. The impurities �particles of dust or ofother substances� present in water and in the walls of thecontainer also play an important role in boiling.6,7 They areboiling centers from which the bubbles of air can easily be-come larger. The impurities locally lower the surface tensionof the water, favoring the expansion of the bubbles.

III. SUPERHEATED WATER

The bubbles produced during boiling are already presentin the water or absorbed in the walls of the container andbecome larger as the temperature increases. The bubbles ofair are the starting point for boiling. Without them boilingcannot occur. If we could eliminate all the air that is in thewater and in the walls of the container, boiling would nothappen. However, this elimination is difficult to accomplish.

It is more feasible to purge the water and its container ofthe largest bubbles, keeping only those that have a very smallradius. In this way the contribution of the curvature pressurepC=2� /r becomes significant. Thus, we could succeed inhaving the water reach a temperature higher than its boilingpoint without boiling. The water in such state is said to besuperheated.6–8

Superheated water is metastable.6 If some metallic filingsare put in the water to create centers of vaporization contain-ing air bubbles, violent boiling will begin with a suddendecrease in the temperature to the characteristic boiling pointtemperature.7

IV. EXPERIMENTAL VERIFICATION OF THESUPERHEATING OF WATER

Superheating can easily be observed in a microwave ovenand is discussed in many books and articles.7–10 It is moredifficult to observe using a stove. To obtain superheated wa-

ter, it is necessary to purge the water of bubbles of air and

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very carefully clean the container. I first cleaned the flaskused to contain the sample of water with ethanol to removethe fat substances off the walls, then with sulfuric acid, andfinally with distilled water. Next, I brought to boil for severalminutes a sample of water in a round-bottomed glass flask tothe normal atmospheric pressure, and then let its temperaturedecrease to around 50 °C. Finally, I boiled the water in a belljar, this time lowering the external pressure. The bell jarallows pressures of the order of 3–4 kPa to be reached.

This procedure was very efficient. When reheating the wa-ter, it reached a temperature of about 108 °C without boiling�see Fig. 1�. As shown in Fig. 2 the temperature had fluctua-tions of about 1 °C. Figures 1 and 2 were created with theon-line acquisition software CassyLab11 employing a

Fig. 1. The water temperature versus time. Note how the water first reaches108 °C without boiling �superheating�. Later, with the addition of iron fil-ings, its temperature quickly reaches the boiling point and stays in theliquid-vapor state.

Fig. 2. Close up of the data in Fig. 1 near the maximum superheating of the

water.

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NiCr-Ni thermometer probe dipped in the sample of water.The temperature averaged over 100 ms was recorded everysecond. Figure 3 shows the experimental apparatus that wasused.

The temperature fluctuations correspond to the formationof large air bubbles that, at the instant they free themselves atthe surface, lower the temperature. This fluctuating state ismaintained for about 5 min. At this point I inserted someiron filings in the water. Immediately the boiling restarted�see Fig. 4� and the temperature decreased to about 99 °C�see Fig. 1�, which is the boiling point at the local atmo-spheric pressure. �The school is not situated at sea level andthe barometer of the laboratory measured a pressure of about98 kPa.�

We can estimate the bubble size. Because the boiling pointincreases 1 °C for an increase of about 3.6 kPa of the satu-rated vapor pressure5 and reached a temperature of around108 °C without boiling, the curvature pressure pC�28 kPacorresponds to a bubble radius of

Fig. 3. Experimental equipment for the water superheating study. You cansee the Bunsen burner with the flask containing superheating water �see themonitor of the computer� and the thermometer probe connected to the inter-face and the computer.

Fig. 4. Boiling caused by the addition of iron filings �right� in superheated

water �left�.

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r =2�

pC�

2 � 7 � 10−2

28 � 103 = 5� m, �4�

which means that we have eliminated all the bubbles of airthat have a radius greater than 5 �m from the water and thewalls of the container.

ACKNOWLEDGMENTS

I would like to thank the Physics Laboratory technicianAngelo Budello, who helped me do the experiments, and myfriend Dr. Giuseppe Curello who helped me translate the textinto English.

1C. R. Nave, “HyperPhysics,” �hyperphysics.phy-astr.gsu.edu/hbase/hframe.html�.

2W. R. Robinson and J. J. Nash, “Visualization and problem solving for

498 Am. J. Phys., Vol. 75, No. 6, June 2007

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general chemistry,” �www.chem.purdue.edu/gchelp/liquids/boil.html�.3J. Walker, The Flying Circus of Physics with Answers �Wiley, New York,1977�.

4“Vapor pressure data of H2O,” �dbhs.wvusd.k12.ca.us/webdocs/GasLaw/Vapor-Pressure-Data.html�.

5Martin Chaplin, “Water structure and behaviour,” �www.lsbu.ac.uk/water�.

6L. Gunther, “A comprehensive treatment of classical nucleation in a su-percooled or superheated fluid,” Am. J. Phys. 71, 351–357 �2003�.

7T. K. McCarthy, “My cup runneth over,” Phys. Teach. 36, 316–316�1998�.

8D. G. Haase, A. Davidescu, G. Kerbaugh, and J. L. Eury, “Simple ex-periment on superheated fluids,” Am. J. Phys. 46, 853–854 �1978�.

9J. Wolfe, “Superheating and microwave ovens,” �www.phys.unsw.edu.au/~jw/superheating.html�.

10M. Vollmer, K. P. Möllmann, and D. Karstädt, “More experiments withmicrowave ovens,” Phys. Educ. 39�4�, 346–351 �2004�.

11 Computer Assisted Science System—LD Didactic GmbH, �www.leybold-didactic.de/data_e/software/index.html?cassy-s.html�.

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