water circulating inside the bowl.
Ultimately, the rate of cooling the core of this body of water becomes so slow that the other
warmer one is always fully frozen first. While there are limitations to this comparison (for
example, we would not see such a result if one quantity were at 10C and another at 990C) this
counter-intuitive result does hold true within the 5–350C range of temperatures indicated
previously.
Since this paper was published, the validity of the research findings has been questioned by a
number of reviewers. They point out that the initial experimental question was not clearly
defined; for example, the researchers needed to decide on exactly what constituted freezing the
water. They also state that the rate at which water freezes depends on a large number of
variables.
Container size is one of these; for the Mpemba Effect to be noticed, the container must be large
enough to allow a free circulation of water to take place, yet small enough for the freezing areas
of the side and base to be effective at extracting heat too. Secondly, research at a University in
St Louis, Missouri, suggests that the Mpemba Effect may be affected by water purity, or by
dissolved gas in the water.
Distilled water is totally free of the particles that are common in normal drinking water or
mineral water. When suspended in water, these particles may have a small effect on the speed
of cooling, especially as ice molecules tend to expel them into the surrounding water, where
they become more concentrated. Just as salt dissolved in water will raise the boiling point and
lower the temperature at which it freezes, the researchers found that the final portion of
ordinary water needed extra cooling, below zero, before all was frozen solid.
One more factor that can distort the effect is observed if the bowls are not placed
simultaneously into the same freezer. In this case, the freezer thermostat is more likely to
register the presence of a hotter bowl than a colder one, and therefore the change in internal
temperature causes a boost of freezing power as the motor is activated.
The Mpemba Effect is still not fully understood, and researchers continue to delve into its
underlying physics. Physicists cannot reach consensus. Some suggest that supercooling1 is
involved; others that the molecular bonds in the water molecules affect the rate of cooling and
freezing of water. A 2013 competition to explain the phenomenon run by the Royal Society of
Chemistry attracted more than 22,000 entries, with the winning one suggesting supercooling
as an important factor so it seems the question and its underlying explanation continue to
fascinate.
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