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Text 2. Supercool theory solves hot ice-cream puzzle
How is it possible for hot water to freeze more quickly than cold? This peculiar phenomenon, first noticed by Aristotle in the 4th century BC, has baffled scientists for generations. But a South African physicist now claims that the answer lies in water’s ability to remain liquid below its normal freezing point. «It’s with supercooling», says David Auerbach, who works at the Ma Institute for Fluid Dynamics in Gottingen, Germany.
The phenomenon is today known as the Mpemba effect, after the Tanzanian schoolboy Erasto Mpemba. In the 1960s, Mpemba became a laughing stock after telling his science teacher he could make ice-cream mixture freeze faster by warming it before putting it into the freezer.
Auerbach turned his attention to the Mpemba effect last year while on sabbatical at the Institute for Water Research in Perth, Australia. He plunged two sealed 100-millilitre beakers - one containing water at 18°C and the other at 90°C - into a bath of cold ethanol and monitored their temperature with a thermistor pasted to the wall of each.
Over the course of 103 experiments, Auerbach found that the Mpemba effect was not a hard-and-fast rule. Sometimes the hot water appeared to freeze first, sometimes the cold. Invariably, however, Auerbach found that the water in each beaker did not start to freeze until its temperature was well below 0°C. When freezing began, branching “ferns” of ice shot out from the walls of the beaker into the water as the temperature jumped back to 0°C. “You can see the same effect when frost forms on the inside of a bedroom window”, says Auerbach. “It’s an unmistakable signature of freezing after supercooling”.
The ice crystals grew to fill the beaker in less than a second, leaving a mush of ice and water. “This is also typical of supercooling”, says Auerbach. “If the water is close to zero when it freezes, the mush is mostly water; if it is much below zero, it is mostly ice”.
Auerbach says that the temperature at which a supercooled liquid starts to freeze is inherently unpredictable. When the Mpemba effect occurs, says Auerbach, it is because the sudden appearance of ice crystals from the supercooled liquid occurs at a higher temperature in the water that was formerly hot. Nevertheless, he says, the apparently solid mass of ice that results is still largely composed of water. Even if the hot water starts to freeze first, the cold water is always the first to freeze completely.
Auerbach found that the Mpemba effect seemed more frequent if the ethanol bath was held between -5°C and -10°C. He has no explanation for this apparent bias, and adds that more experiments are needed to confirm that it was not a statistical fluke. If the bias is real, however, it could explain why Mpemba’s hot ice-cream mixture usually appeared to freeze first.
Marcus Chown. New Scientists, 1995
Vocabulary and Comprehension Exercises
I. Translate these into Russian:
• the peculiar phenomenon
• below its normal freezing point
• a sealed beaker
• to monitor the temperature
• a thermistor pasted to the wall
• branching “ferns” of ice
• an unmistakable signature of freezing
• a mush of ice and water
• to be unpredictable
• the sudden appearance of ice crystals
• to freeze completely
• an apparent bias
• to be a statistical fluke
II. State the part of speech of the following words and translate them into your own language:
• supercool, unmistakable, unpredictable, unreal, impossible, to refreeze, to disappear;
• invariably, completely, inherently, formerly, largely.
III. Give the plurals of the nouns below:
phenomenon, medium, formula, basis, datum, momentum, analysis, nucleus.
IV. Translate the sentences paying attention to the underlined words.
• This peculiar phenomenon, first noticed by Aristotle in the 4th century BC, has baffled scientists for generations.
• In the 1960s, Mpemba became a laughing stock after telling his science teacher he could make ice-cream mixture freeze faster by warming it before putting it into the freezer.
• Auerbach plunge two sealed beakers-one containing water at 18°C and the other at 90°C - into a bath of cold ethanol and monitored their temperature with a thermistor pasted to the wall of each.
4. Sometimes the hot water appeared to freeze first, sometimes the cold.
V. Complete the sentences choosing the proper words from the given below and arrange them in a logical order according to the text.
phenomenon; supercooling; the Mpemba effect; appeared; to freeze; unpredictable; hard-and- fast rule; beakers; temperature; appearance
• Over the course of 103 experiments Auerbach found that the Mpemba effect was not a ..... .
• This peculiar ..... was first noticed by Aristotle in the 4th century BC and is today known as ..... .
• Auerbach says that the temperature at which a supercooled liquid starts to freeze is inherently ..... .
• How is it possible for hot water ..... more quickly than cold?
• “It’s all to do with .....”, says David Auerbach.
• Sometimes the hot water ..... to freeze first, sometimes the cold.
• He plunged two sealed ..... - one containing water at 18°C and the other at 90°C - into a bath of cold ethanol and monitored their ..... .
• When the Mpemba effect occurs, says Auerbach, it is because the sudden ..... of ice crystals from the supercooled liquid occurs at a higher temperature in the water that was formerly hot.
VIII. Answer the questions:
• What phenomenon, first noticed by Aristotle, has baffled scientists for generations?
• How is it called today? Why?
• What is the opinion of David Auerbach?
• Can you describe the experiments carried out by Auerbach?
• What did he find over the course of 103 experiments?
• What does Auerbach say about the temperature at which a supercooled liquid starts to freeze?
• When does the Mpemba effect occur according to Auerbach?
• When did the Mpemba effect seem more frequent?
• Is there any explanation for this apparent bias?
IX. Write a summary in English (or in your own language).
• Give each paragraph a suitable title in English (or in your own language).
• Develop the titles of the paragraphs into topic sentences. Join the topic sentences together.
• Re-read your summary and make sure that the sentences are
presented in a logical order.
Additional Text. Read the following information and comment on it.
“The Last Word”
Q: Is it true that hot water placed in a freezer freezes faster than cold water?
And if so why does this happen?
This question was raised many years ago in New Scientist and never answered satisfactorily. This time we are closer to settling the controversy with answers from several people who have tried the right experiments. Counterintuitive though it may be, it does appear that hot water can freeze more quickly in a refrigerator. Better thermal contact if the water container is placed into an iced-up freezer compartment and a different pattern of convection currents which allow hot water to freeze faster seem the best explanations. Which effect predominates depends on the fridge, the container and where it is placed.
A. 1 : The questioner is correct - it is possible to produce ice cubes more quickly by using initially hot water instead of cold. The effect can be achieved when the container holding the water is placed on a surface of frost or ice. The higher temperature slightly melts the icy surface on which the container rests, greatly improving the thermal contact between the container and the cold surface. The increased rate of heat transfer from the container and contents more than offsets the greater amount of heat that has to be removed. The effect cannot be obtained if the container is suspended or rests on a dry surface.
This effect was first noted by Sir Francis Bacon using wooden pails on ice. My own investigation showed ice cubes could be obtained within 15 minutes rather than 20 minutes if the frost in the refrigerator was deep enough. The incentive to get your ice a little quicker is obviously greater in Australia than in cooler countries.
Michael Davis. University of Tasmania.
But Sir Francis Bacon was not the first to note the effect. Aristotle’s account in Meteorology below implies a similar explanation:
A: “Many people, when they want to cool water quickly begin by putting it in the sun. So the inhabitants when they encamp on the ice to fish (they cut a hole in the ice and then fish) pour warm water round their rods that it may freeze the quicker; for they use ice like lead to fix the rods”.
David Edge. Hatton, Derbyshire.
And it seems untrue that the “effect cannot be obtained if the container is suspended or rests on a dry surface .....”
A. 3: It’s true and I have verified the assertion by experiment. The only limitation is that the container of water must be relatively small so that the capacity of the freezer to conduct away the heat content is not a limiting factor.
Cold water forms its first ice as floating skin, which impedes further convective heat transfer to the surface. Hot water forms ice over the sides and bottom of the container, and the surface remains liquid and relatively hot, allowing radiant heat loss to continue at a higher rate. The large temperature difference drives a vigorous convective circulation which continues to pump heat to the surface, even after most of the water has become frozen.
Tom Hering. Kegworth, Leicestershire.
A. 4: This is a cultural myth. Hot water will not freeze faster than cold water in the freezer. However, hot water cooled to room temperature will freeze faster than water that has never been heated. This is because heating causes the water to release dissolved gases (mostly nitrogen and oxygen) which otherwise reduce the rate of ice crystal growth.
Tom Trull. University of Tasmania.
After having read “Supercool theory solves hot ice cream puzzle” and the additional text try to express your own opinion on the subject.