LA PHYSIQUE ET L’ÉDUCATION ( A STRANGE BEHAVIOUR OF WATER )
Editorial Note -- The following paper is being published in the Physics and Education Section of this journal because the paper presents the
results of a very interesting project on what has been called the Mpemba effect, in which hot water appears to freeze more quickly than cool water.
The author is to be congratulated on carrying out a very careful series of experiments as part of a high school science project. The results of this
work should certainly be of interest, as the paper contains sufficient new physics for this section of Physics in Canada. It should be noted, however,
as pointed out by C. A. Knight (Am. J. Phys. 64, 524 (1996)] that the recent literature on the Mpemba effect has overlooked a very thorough and
comprehensive study published by N. E. Dorsey [Am. Philos. Soc. 38, 247-328 (1948)]. This in fact predates Mpemba’s observations by 15 years.
Dorsey’s main conclusion was that the nucleation of freezing was caused by “motes”; i.e. very tiny particles of foreign matter in the water. He
found that heating the water served to deactivate supercooling before nucleation. Even though the present work is very well done, and an
important accomplishment for the students involved, it is not clear that it adds substantially to what has already been published by Dorsey.
However, for general interest, and as a stimulus to others to examine this effect, the author has offered the paper for publication in Physics in
Canada. NOTE: Some editorial modification to both the language and the presentation of this paper has been undertaken. J.S.C. McKee, PPhys
A STRANGE BEHAVIOUR OF WATER
by Concetto Gianino
I
NTRODUCTION
Mr. Angelo Budello and Mr. Franco Mazza helped us
throughout the research.
This phenomenon, known as the
THE PROJECT MARLIANIIf you cool two identical
Mpemba effect, is certainly strange
MPEMBA
and almost unbelievable. The most
samples of water, one of
The project was born from the idea of
common proof used against it
the Internet as a learning
sounds like this: the initially warm
which is at a higher temper- using
instrument,
where it was possible to
water has to take some time cooling
ature
than
the
other,
which
cooperate,
to
carry out researches, to
to get down to the temperature of
set
up
and
spread
information, and to
the initially cold water, which in the
of them will freeze first?
solve
problems.
The
use of a web
meantime will get down to a lower
communicative
platform
of the kind
The
answer
can
seem
quite
temperature till freezing before the
24-7,
Virtual
Classroom,
that
other one. For this reason the
simple and obvious: the ini- 24 hours a day for seven daysworks
a week,
Mpemba effect becomes impossible.
allowed
us
to
exploit
all
the
web
tially
cooler
water.
However
In other words the initially cold
interactive techniques, involve
water has no reason to wait for the
it has been observed that
students in the learning process
initially warm water to cool before it
(Engaged Learning) and fulfil a sort
sometimes
the
initially
does. The fundamental assumption
of distributed, constant and enduring
of the proof above is that the two
warmer water freezes faster learning (Distributed Learning).
samples of water are equivalent unit
than the initially cold one.
systems from a thermodynamic,
The project was carried out by a
chemical and physical point view.
research – action approach. The use
But is that true? May the heating
of the instruments offered by the
modify the chemical and physical conditions of water? For
Internet enabled us to reach different didactic aims. In
example is the content of gases and mineral salts equivalent
particular our students have acquired new knowledge and
in the two samples of water? Are we sure that water freezes
specific abilities and competences when:
at 0° C? May water remain in a liquid state even at temperatures below 0° C under normal atmospheric pressure?
- using instruments of asynchronous (personal emails and
mailing list) and synchronous (chatting) interaction in the
In order to answer these questions, during the school year
Internet;
2002/2003, I started a project, called Marliani-Mpemba, on
- carrying out researches by search engines;
the Mpemba effect. I worked with my colleague
- working on the web and sharing it with different users;
Nicolantonio Cutuli from the Liceo Scientifico Statale
- producing multimedia documents and collecting them in a
“G. Berto” in Vibo Valentia on this project.
CD with hypertexts;
- using the scientific method in real and concrete research;
The project was developed using the Internet as a means of
- reproducing the phenomenon of supercooling in the
cooperation and involved the students of class IV A, of the
laboratory;
Istituto di Istruzione Secondaria Superiore “Q: Cataudella”
- preparing a freezing mixture NaCl-ice and understanding
where I teach and those of the IV B and the IV F of the Liceo
the process of cooling caused by the addition of salt to the
“G: Berto”. Results were checked by two experts:
ice;
Ms. Giuseppina Rinaudo, a professor at the Physics
Department in Turin and Mr. Antonino Foti, a professor at
Concetto Gianino, Istituto di Istruzione Secondaria
the Physics Department in Catania. Two technical assistants
Superiore “Q.Cataudella” – Viale dei Fiori 13, 97018
of the physics laboratories in our schools:
Scicli (RG)
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PHYSICS AND EDUCATION ( A STRANGE BEHAVIOUR OF WATER )
- understanding the mechanisms leading to freezing;
- understanding the role of evaporation in the cooling of a
liquid
- and, certainly, studying the Mpemba effect.
The project was developed in the virtual working
background proposed by Yahoo Groups and involved three
different and fundamental groups of participants: the
coordinators, the experts and the actors. The actors, that is
our students, had the task of carrying out the research and
supplying information with documentary evidence and
comments. In their tasks they were guided by the
coordinators (their teachers) and checked by the experts (the
professors). The students were subdivided into six
subgroups (named from A to F). Each of them had a lead
student and the fixed task to look for specific, agreed
information in the web, later to be drawn up in a written
report. The task of each subgroup was set up as follows:
- Subgroup A: historical references;
- Subgroup B: the role of cooling evaporation;
- Subgroup C: the contribution of dissolved gases;
- Subgroup D: the contribution of convection currents;
- Subgroup E: the phenomenon of water supercooling;
- Subgroup F: analysis of the surrounding environment.
The project also included different experimental moments
during which the students, guided by their teachers, with
the collaboration of the lab technicians and advised by the
professors, had to test the quality of the phenomenon under
a wide range of circumstances.
In addition to the quality experiments, some quantitative
experiments were carried out both by means of traditional
lab devices and by means of new devices on-line.
As our project had innovative elements both in its didactic
use of the Internet and in its production of some original
results, it has been chosen from among national projects to
join the third edition of the European “Physics on stage”
held at ESTEC in Noordwijk in Holland from 8th to 15th
November, during the European week of Science and
Technology established by the European commission. In
addition, our project was distinguished as being the first in
Calabria among the didactic experiences of “best practice”
of the Italian documentary school system, given a GOLD
award and promoted by INDIRE.
The positive results of the project led my school, the
Institute “Q. Cataudella” and the Liceo “Berto” to agree to a
permanent cooperation on-line. This new project, named
DRAGO (Didattica in Rete Applicando i Gruppi On-line,
means education in networking with on-line groups), and
will be concerned with the study of annual scientific topics.
THE HISTORY OF THE EFFECT MPEMBA
Even Aristotle knew that hot water might freeze faster than
cold water. In 300 BC he referred to this phenomenon in his
“Meteorologica I”, trying to explain it with the idea of
antiperistasis that is “the increase in the intensity of a
quality as a result of being surrounded by its contrary
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quality, for instance, the sudden heating of a warm body
when surrounded by cold”.
In 1461 Giovanni Marliani confirmed the strange
phenomenon by placing four ounces of boiling water and
four ounces of non-heated water outside on a cold winter
day.
Some centuries later, in 1620, the English humanist Francis
Bacon wrote about the phenomenon in his “Novum
Organum” and a little later Rene Descartes did the same.
The modern scientific community generated renewed
interest in the effect in 1969, thanks to an event in the life of
a Tanzanian high school student named Mpemba. The event
provides us with a dramatic example of the uncareful
consideration with which teachers often dismiss their
students’ observations and make quick judgements about
what seems them to be impossible. In 1963 the young
Mpemba, together with his classmates, had to prepare ice
cream with some sweet milk. He, unable to get scarce
refrigerator space, put his hot ice cream in the fridge
without cooling it before. To his surprise he found that his
ice cream had frozen before the other students’. He got
curious about that and repeated the process with the same
result. But, when Mpemba asked his physics teacher for an
explanation, he was carelessly answered: “it can’t be, you
must have been mistaken”; when he insisted on his
observations, he was replied, “these events occur only in
Mpemba’s physics, not in the real world”.
Later, on the occasion of a visit of Dr. Osborne, a professor
of Physics at Mpemba’s high school, the young student was
brave enough to ask him the reason for the strange
phenomenon he had observed. Dr. Osborne answered that,
at the moment, he was not able to think of any explanation,
but that he would try the experiment later. Dr. Osborne’s
repeated tests gave the same positive results, which he
wrote up in 1969. In the same year Dr. Kell wrote a paper
about the phenomenon independently and unaware of
Osborne’s experiments. From then on several experiments
have confirmed the Mpemba effect.
The most recent papers on the effect are a publication,
which dates back to 1995. The author, David Auerbach,
assumes that the supercooling of water is the fundamental
explanation of the phenomenon.
POSSIBLE EXPLANATIONS OF MPEMBA EFFECT
We have realized, through our research and the project
Marliani-Mpemba that “the mechanism” explaining the
Mpemba effect has not been found yet, but that different
mechanisms contribute to it under different conditions.
They are:
- cooling by evaporation: evaporation is certainly more
significant in the initially warmer water than in the
initially cooler water. This process, not only makes warmer
water cool easier, but also determines a mass reduction
with the consequent decrease of heat capacity. Evaporation
might explain the Mpemba effect when the cooling
evaporation is the only or at least the most important
LA PHYSIQUE ET L’ÉDUCATION ( A STRANGE BEHAVIOUR OF WATER )
- convection currents: if we analyse the process of the
cooling of water in detail, we can affirm that, probably, the
temperature of the sample of water is non-uniform. In fact
the surface of the water loses heat more quickly, because of
evaporation, developing a temperature gradient and
convection currents. Convection currents will be more
relevant in the initially warmer water than in the initially
cold water. Therefore the initially cold water will be able to
form a top surface of fast forming ice. This will work as
insulation, will slow down the process of cooling and could
make the Mpemba effect appear. Even in this case, some
experiments observed the Mpemba effect, but no ice was
seen on the surface of the initially cold water.
Fig. 1 The curve of the cooling of a 10 ml sample of water
soaked, by means of a bar, in a freezing mixture of ice
and sodium chloride at - 18° C. the temperature of the
water in the bar was checked by a thermometric probe
NiCr-Ni connected to the computer through a digital
interface thermometer.
cooling mechanism. However the phenomenon has been
observed even when evaporation had not been so massive;
- dissolved gases: the initially hot water contains less
dissolved gas, in particular less O2 and CO2 than cold
water, as large amounts of gas escape upon boiling. It has
been supposed that, in some way, this condition changes
the properties of water, perhaps making it easier to
develop convection currents (and thus making it easier to
cool), or decreasing the amount of heat necessary to freeze
a unit mass of water or change the boiling point.
Unfortunately the experiments, which favour this
explanation, aren’t supported by exhaustive theoretical
calculations.
In our quality analysis we used drinking water to test if any
of these effects played a predominant role and we repeated
the experiments where one or two of the above conditions
were prevailing. We used different containers: some were
open, metal, containers which warranted a more uniform
distribution of temperature, so they should have reduced
convection currents; some were open, plastic and glass
containers where cooling evaporation and convection
currents should have prevailed; some others were closed to
reduce the effect of cooling evaporation.
- supercooling: the supercooling of water is another
interesting phenomenon that also came out in our
research. In the lab we tested that, under
the normal atmospheric pressure, water
can easily cool below 0° C without
freezing. First we tried with distilled
water, which we easily froze down to 7° C, then we used some drinking water,
we froze it down at temperatures below 12° C (see Figure 1). In short at 0° C water
molecules, despite their energy state
might arrange them as an ice crystal, are
not able to form themselves as a solid ice
lattice, unless they find a nucleation site
to tell them how to rearrange themselves.
Supercooled water is in a state of
instability and the insertion of a simple
jolt or an ice lattice, like a grain of ice or a
speck of dust, can determine a change in
its state (see Figure 2). This phenomenon
might be the explanation of the freezing
of the initially warm before the initially
cold water, as the freezing from the state
of supercooling is random and
unpredictable. Indeed Auerbach’s
experiments stressed that from the phase
of supercooling the initially hot water
Fig. 2 Sequences of the cooling of some water from the state of supercooling by
would freeze more than the initially cold
means of a grain of ice which can be seen in the sequences a and B next to
water;
the interface water/air in the bar. The water in the bar is distilled at - 7°C.
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PHYSICS AND EDUCATION ( A STRANGE BEHAVIOUR OF WATER )
Furthermore some
samples of water
were boiled, so that
some gases escaped
upon boiling; others
were simply heated
without boiling them.
We also tried to
analyse the cooling of
the samples of water
with some gases
added to them.
These experiments
pointed out that the
effect exists, but it
does not always
appear even when
you follow the same
procedure and, when
it appears, it does not
seem that there is a
predominant
mechanism, but it
seems likely that
different conditions
and mechanisms
contribute to the
effect.
Fig. 3
An experimental device used to study the
Mpemba effect.
THE RESULTS OF THE QUANTITATIVE ANALYSIS
After the quality analysis we undertook a range of
quantitative tests. In summer I, helped by the lab technician,
Mr Angelo Budello, undertook these tests in the physical
laboratory of my school the Istituto “Q. Cataudella”. The
samples of warm and cold water were put into two lab bars,
both soaked in a freezing mixture NaCl-ice at – 18° C. The
freezing mixture was in two cylindrical vessels made of
aluminium and placed in a polystyrene container (see
Figure 3). We used two 20 ml syringes to withdraw the
same quantity of water from both samples.
The temperature of the samples of water and of the freezing
mixture was checked by means of an on-line real-time
system by Leybold, consisting of 4 thermometric probes at
NiCr-Ni, connected through a digital thermometer to the
serial port of a computer. In each experiment we heated
about 200 gr. of water till it boiled. From it we withdrew the
sample of hot water to be inserted in one of the two bars.
We kept boiling for about one minute, then, when the
temperature of water got to about 80° C, we withdrew a
10 ml sample of water by means of the syringe, at the same
time we withdrew the same quantity of water from the
ambient temperature sample. The two samples were then
injected into the bars, which had previously been placed
into the freezing mixture, and then we started the computer
to acquire the data.
As we were operating in pairs, some seconds went by from
the moment of the insertion of the samples into the bars and
the beginning of the data acquisition. Just as had happened
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during the Fig. 4 The curves of cooling of two samples of
quality
water of which one was hot and initially
experiboiled, while the other was at about 30°
ments,
C, both placed in a container at - 18° C.
even
You can notice the Mpemba effect (see
the arrow) and a strange phase transition
during
of the initially colder water at about 4° C
these tests
(see the circled area).
the
Mpemba
effect did not always occur, although we always proceeded
in the same way and used similar samples of water. On the
contrary the supercooling both of the initially warm water
and of the water at the ambient temperature was a
phenomenon that we always observed. This has confirmed
that water can easily supercool.
The Mpemba effect appeared only twice: the first time it
was clear and plain (see picture 4); the second time the
cooling of the initially warm water appeared only a couple
of seconds before the other one (see pictures 5 and 6). In
almost all the experiments a new common phenomenon
was the anomaly in the cooling curve of the water at the
ambient temperature. There was a slowing-down of the
cooling process at about 3°/4° C which, in some cases,
became an isothermal process typical of phase transitions.
This phenomenon was essential to the appearance of the
Mpemba effect in our experiments. In fact, as you can see in
Figures 4 and 5, during that strange phase transition, the
initially cold water, actually, “waited for” the initially warm
water.
What happened to the sample of water at a temperature
between 3° C and 4° C?
LA PHYSIQUE ET L’ÉDUCATION ( A STRANGE BEHAVIOUR OF WATER )
The strange effect appears just in that temperature band in
which water gets the highest density and later, after cooling,
it expands instead of contracting.
The hypothesis that the effect is produced by the
development of a frozen top on the internal surface of the
bar must be rejected because, in this case, water could not
supercool as in the experiment in Figure 6, and there would
be some nucleation.
BUT WHY DOES IT HAPPEN ONLY WHEN WE USE
UNBOILED WATER?
The initially cool water has more dissolved gas than warm
water. Can we think that the larger quantity of molecules of
O2 and CO2 works as a mediator to set up hydrogen bonds?
In fact at the temperature of 4° C water molecules should be
at the average minimal distance and hydrogen bonds could
be easily made. Another hypothesis to be tested could be the
forming of precipitate.
No special effect appeared between 3° C and 4° C during the
only test we have effected. In any case, only one test alone is
not obviously enough to get to well-agreed, unquestionable
explanations, but it is necessary to repeat the experiment
several times to test its long-term occurrence, not its casual
happening.
THANKS
Many thanks to my colleague and friend Nicolantonio
Cutuli for his help with this text and also to all members of
the group, who worked on the project with willingness and
zeal. Thanks to my colleague, Maria Ficili for her linguistic
collaboration.
Finally, my special thanks to Professor Giuseppina Rinaudo
for our useful and discussions online and to Mr Angelo
Budello for his invaluable help during the work in the
physics laboratory.
Fig. 5
The curves of cooling of two samples of water.
You can notice the Mpemba effect and also the
strange phase transition of the initially colder
water at about 4° C.
REFERENCES
M. Jeng, Can hot water freeze faster than cold water?
http://math.ucr.edu/home/baez/
physics/General/hot_water.html; Hot Water
Freezing?, http://hyperphysics.phyastr.gsu.edu/hbase/thermo/freezhot.html#c3; Is
it true that hot water placed in a freezer freezes
faster than cold water?, http://www.newscientist.com/lastword/ article.jsp?id=lw236; The
Mpemba effect , http:// www.school-for-champions.com/science/mpemba.htm; Thirty eight
anomalies of water1, http://www.martin. chaplin.btinternet.co.uk/anmlies.html; Surfusion,
http://cyberzoide.developpez.com/surfu.
E.B.Mpemba and D.G.Osborne, “Cool?”, Physics
Education, 4, No 3 (May 1969), pp.172-175.
Kell G S, “The Freezing of Hot and Cold Water”
American Journal of Physics, v.37, p.564-5, May
1969.
D. Auerbach, “Supercooling and the Mpemba
Effect: When hot water freezes quicker than
cold”, American Journal of Physics, Vol. 63, Issue
10, pp. 882-885, (October 1995).
Fig. 6
A detail of Figure 3, where the Mpemba Effect is better pointed
out.
C. Gianino, “L’effetto Mpemba”, Giornale di Fisica,
Vol. XLV, n.1, gennaio-marzo 2004.
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