READING PASSAGE 3
You should spend about 20 minutes on Questions 27-40
Questions 27-40, which are based on Reading Passage
3.
THE MPEMBA EFFECT
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, wh
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