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The Mpemba effect (im-PEM-bah). When Hot Water Freezes Before Cold Water. James D. Brownridge Department of Physics, Applied Physics and Astronomy State University of New York at Binghamton Binghamton University. A Brief Introduction. - PowerPoint PPT Presentation
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The Mpemba effect(im-PEM-bah)
When Hot Water Freezes Before Cold Water
James D. Brownridge
Department of Physics, Applied Physics and Astronomy
State University of New York at Binghamton
Binghamton University
An experimental explanation for why hot water will sometimes freeze more rapidly than cold water is offered. Two specimens
of water from the same source will often have different spontaneous freezing temperatures; that is, the temperature at which freezing begins. When both specimens supercool and
the spontaneous freezing temperature of the hot water is higher than that of the cold water, then the hot water willusually freeze first, if all other conditions are equal and
remain so during cooling.
A Brief Introduction
Under normal conditions, ice that is warmed from less than 0o C will always begin melting when its temperature reaches 0o C. However, when liquid water is cooled from
above 0o C, it often will not begin freezing until it has supercooled to several degrees below 0o C. This is why
hot water can freeze before cooler water when all experimental conditions are identical except for the initial temperatures of the water. Hot water will freeze before cooler water only when the cooler water supercools, and then, only if the nucleation temperature of the cooler
water is several degrees lower than that of the hot water.
Freezing and melting
Heating water may lower, raise or not change the spontaneous freezing temperature. The keys to
observing hot water freezing before cold water are supercooling the water and having a significant
difference in the spontaneous freezing temperature of the two water specimens.
Heating water
Freezing water
Clean water that is setting undisturbed in a freezing environment (freezer) will not freeze when its temperature falls to 0o C. It will supercool to well below 0o C before
heterogeneous nucleation initiates freezing. Small volumes of very pure water with no “ice nucleation agents” (foreign
agents) can be supercooled to ~ -40o C; at this temperature it is homogeneous nucleation that initiates freezing.
When freezing is initiated by heterogeneous nucleation the water will freeze when its temperature reaches the “ice
nucleation temperature” of the foreign agent with the highest “ice nucleation temperature”
Experimental Results
Most outstanding cases of hot water freezing first
(Slide 7, 8 and 9)
A few silver iodide crystals were added to 3 of 6 vials
Experimental set-up
DAQ
Up to 8 vials
Experimental set-up
DAQ
Up to 8 vials
0 20 40 60 80 100 120 140-20
-10
0
10
20
30
40
50
60
70
80
#3
Vials 3 & 4Vials 3 & 4T
rmpe
ratu
re o C
Vials 1 & 2
Last to freeze
Hot 1st to freeze
0 20 40 60 80 100 120-20
-10
0
10
20
30
40
50
60
70
80
90
Hot 1st to freeze#3 Last to freeze
Vials 1 & 2
0 20 40 60 80 100 120-20
-10
0
10
20
30
40
50
60
70
80
Trm
pera
ture
o C
Cold 1st to freeze Cold 1st to freeze
#3
#2 Last to freeze
Vials 3 & 4
Vials 1 & 2
Time (min)
0 20 40 60 80 100 120 140-20
-10
0
10
20
30
40
50
60
70
80
Last to freeze
4 "identical" glass vials; Nucleation agents unknown
Vials 1 & 2
Vials 3 & 4
Time (min)
#3
Experimental set-up
Top thermocouple
Bottom thermocouple
Hot water freezing before very cold water
“Spontaneous ice-nucleation temperature”
HOT COLD
Top Thermocouple
Bottom thermocouple
Added
“Spontaneous ice-nucleation temperature site”
*
*
If T=0 At 35 msec At 70 msec At 105 msec
freezing
The speed of the ice front depends on how low the water is supercooled
Photos of freezing water (2 ml of water)
15 freeze/thaw cycles
-10.8±0.3
15 freeze/thaw cycles
~70 hrs.
Loss of water apparent
-14.3o C the vial was cracked by the ice during the 1st freeze cycle
[-14.3o C] The vial was cracked by the ice during the 1st freeze cycle
-11.0 ± 0.5 -3.3 ± 1.7 -10.5 ± 0.9 -9.1 ± 0.8 -9.9 ± 0.3 -7.5 ± 1.2 -3.4 ± 1.7 -9.8 ± 0.8
-11.3 ± 0.3 -13.8 ± 0.1 -14.3 ± 0.1 -11.3 ± 0.1 -10.8 ± 0.3 - 9.7 ± 0.1 -1.2 ± 1.1 -9.9 ± 0.2
0.3 ± 0.6 10.6 ± 1.7 3.8 ± 0.9 2.2 ± 0.8 0.9 ± 0.4 2.3 ± 1.2 -2.2 ± 2.0 0.1 ± 0.8
Initially After shaking Net differenceVial #
12345678
Shaking water in a container maychange the
“spontaneous ice-nucleation temperatures”
Latent heat released at oC
13 cycles 13 cycles
0 5 10 15 20 25 30 35 40 45 50 55-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
A spontaneous change in the ice-nucleation temperatures.
Reason unknown.
Vial shaken
Freeze/thaw cycle
Late
nt h
eat r
elea
sed
at o C
Vial # 2
Graphic display of the ice nucleation temperatures
Before shaking After shaking
0 5 10 15 20 25 30 35 40 45 50 55-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
Freeze/thaw cycle
Late
nt h
eat r
elea
sed
at o C
Vial shaken
Vial # 8
0 5 10 15 20 25 30 35 40 45 50 55-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
A spontaneous change in the ice-nucleation temperatures
La
tent
hea
t rel
ease
d at
o C
Freeze/thaw cycle
Vial shaken
Vial # 5
0 5 10 15 20 25 30 35 40 45 50 55-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
5.1±1.316.1±0.2
Late
nt h
eat r
elea
sed
at o C
Freeze/thaw cycle
Vial # 1
Vial shaken ?
0 5 10 15 20 25 30 35 40 45 50 55-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
15.1±0.4
Vial shaken
Late
nt h
eat r
elea
sed
at o C
Freeze/thaw cycle
Vial # 6
15.1±0.1
Graphic display of the ice nucleation temperatures
Hot water never cools to 0oC first
We have determined that when water is added to a container there may be many “ice nucleation sites” with different nucleation temperatures. The temperature of a given site
can often be changed by heat, stirring or jostling water inside the container. Heating water may lower, raise or not
change the spontaneous freezing temperature, however, heating water will not necessarily cause it to freeze faster
than water that was not heated. The nucleation temperature of the “ice nucleation agent” in a sample of water is
responsible for the temperature at which the sample will freeze
Conclusion
Acknowledgments
• I wish to thank Cara Walkin, Julie Galluccio and Mark Stephens for
comments and suggestions.
• 4 March 2010