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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.

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slide1

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

slide2

A Brief Introduction

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 will

usually freeze first, if all other conditions are equal and

remain so during cooling.

slide3

Freezing and melting

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.

slide4

Heating water

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.

slide5

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 nucleationinitiates freezing. Small volumes of very pure water with no “ice nucleation agents” (foreign agents)can besupercooled to ~ -40o C; at this temperature it is homogeneous nucleation that initiates freezing.

When freezing is initiated by heterogeneous nucleationthe water will freeze when its temperature reaches the “ice nucleation temperature” of the foreign agent with the highest “ice nucleation temperature”

slide6

Experimental Results

Most outstanding cases of hot water freezing first

(Slide 7, 8 and 9)

slide10

Experimental set-up

Up to 8 vials

slide11

Experimental set-up

Up to 8 vials

slide13

Experimental set-up

Top

thermocouple

Bottom

thermocouple

slide14

“Spontaneous ice-nucleation temperature”

HOT

COLD

Added

Bottom

thermocouple

Top

Thermocouple

Hot water freezing before very cold water

slide16

Photos of freezing water (2 ml of water)

The speed of the ice front depends on how low the water is supercooled

freezing

If T=0

At 70 msec

At 105 msec

At 35 msec

slide18

-10.8±0.3

15 freeze/thaw cycles

slide19

Loss of

water apparent

[-14.3o C] The vial was cracked by the ice during the 1st freeze cycle

~70 hrs.

-14.3o C the vial was cracked by the ice during the 1st freeze cycle

slide20

Shaking water in a container may

change the

“spontaneous ice-nucleation temperatures”

Latent heat released at oC

Vial #

Initially

After shaking

Net difference

1

2

3

4

5

6

7

8

-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

13 cycles

13 cycles

slide24

Conclusion

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

acknowledgments
Acknowledgments
  • I wish to thank Cara Walkin, Julie Galluccio and Mark Stephens for comments and suggestions.
  • 4 March 2010