雅思阅读真题+题目+答案：the mpemba effect

雅思阅读真题+题目+答案：the mpemba effect

雅思阅读真题原文如下：

In 300 BC, the famous philosopher Aristotle wrote about a strange phenomenon that he had observed: “Many people, when they want to cool water quickly, begin by putting it in the sun.” Other philosophers over the ages noted the same result, but were unable to explain it.

In 1963, a young Tanzanian student named Erasto Mpemba noticed that the ice cream he was making froze faster if the mix was placed in the freezer while warm than if it were at room temperature. He persisted in questioning why this occurred, and eventually physicist Denis Osborne began a serious investigation into what is now known as the Mpemba Effect. He and Mpemba co-authored a paper in New Scientist in 1969, which produced scientific descriptions of some of the many factors at work in freezing water.

It was initially hypothesised that the warm bowl melted itself a place in the ice on the freezer shelf, thus embedding its base in a ‘nest’ of ice, which would accelerate freezing. The hypothesis was tested by comparing the result when bowls of warm water were placed on ice and on a dry wire shelf; this demonstrated that the ice nest actually had little effect. A second suggestion was that the warmer water would be evaporating at its surface, thus reducing the volume needing to be frozen, but this idea was also shown to be insignificant.

Thermometers placed in the water showed that the cooler water dropped to freezing temperature well before the warmer bowlful, and yet the latter always froze solid first. Experiments at different temperatures showed that water at 50C took longest to freeze in a conventional freezer, while water initially at 350C was quickest. On further examination, an explanation for this paradox began to emerge. Losing heat from the water occurs at the points where it is in touch with the colder atmosphere of the freezer, namely the sides of the bowl and the water surface.

A warm surface will lose heat faster than a cold one because of the contrast between the temperatures; but of course there is more heat to be lost from one bowl than the other! If the surface can be kept at a higher temperature, the higher rate of heat loss will continue. As long as the water remains liquid, the cooling portion on top will sink to the bottom of the bowl as the warmer water below rises to take its place. The early freezing that may occur on the sides and base of the container will amplify the effect.

The bowl that is more uniformly cold will have far less temperature difference so the water flow will be minimal. Another inhibiting factor for this container is that ice will also form quite quickly on the surface. This not only acts as insulation, but will virtually stop the helpful effects of the water circulating inside the bowl.

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Questions 1-7

Write the correct letter, A–O, in boxes 1-7 on your answer sheet.

For more than 2000 years people have wondered why raising the 1 ______ of cold water before cooling it results in more rapid cooling. At first researchers thought that a warm container created its own icy 2 ______ which made the water freeze faster, but comparisons with containers resting on a dry 3 ______ indicated that this was inaccurate. Evaporation of water proved not to be a 4 ______ .

Temperature measurements showed that, although the water in the cooler container reached 00C before the warmer one, it took longer to actually solidify. The water temperature drops the most at the top and sides of the container. Provided there is a temperature 5 ______ , the water will continue to circulate and to cool down. Cooler water will have less water 6 ______ , and thus a slower rate of freezing. If ice forms on the top of the water, this will further slow the 7 ______ of freezing, but if it forms on the bottom and the sides of the container, this will increase the rate of cooling.

A. melt

B. element

C. process

D. centre

E. acceleration

F. surface

G. factor

H. hollow

I. matter

J. circulation

K. limit

L. significance

M. theory

N. difference

O. result

P. temperature

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