|
When
faced with an icy car windscreen most of us tend to use warm water
from the kettle to melt the ice rather than cold water from the
cold tap.
Common
sense says that cold water will freeze faster than warm water. But
it was Einstein who once said: "'Common sense is the collection
of prejudices acquired by age eighteen".
As
with so many other things, Einstein was proved right as, although
the phenomenon was described by Aristotle, Bacon and Descartes,
it only became known to the modern scientific community by the enquiring
mind of a 15-year-old Tanzanian schoolboy.
That
hot water freezes at a faster rate than cold water cannot be
disputed. The difficulty is in
explaining exactly why the warmer water freezes first. |
| Richard
Angwin |
That
hot water freezes at a faster rate than cold water cannot be disputed.
Fill two identical containers with the same amount of water and
place them in a freezer. The container filled with the warmer water
will freeze first. The difficulty is in explaining exactly why
the warmer water freezes first.
The
phenomenon had been identified by Aristotle in 300 BC and confirmation
followed from Marliani in 1461, Bacon and Descartes in the 1620s.
The
story continues in a Tanzanian high school in 1969 where a schoolboy
by the name of Mpemba was making ice cream by mixing boiling water
and sugar.
He
had been instructed to allow the mixture to cool before placing
it in the school refrigerator. To avoid a rush for space in the
fridge he ignored the instruction and placed the hot mixture inside.
To his surprise he found that his ice cream mix froze before that
of his fellow students.
Impossible
When
Mpemba asked his physics teacher for an explanation he was told
he must have been mistaken - his results were impossible.
Mpemba
believed his teacher at the time but less than a year later he met
a friend who made and sold ice cream commercially. He confirmed
Mpemba's observations.
Things
might have rested there but for the visit to Mpemba's school of
a professor of physics. Mpemba asked the professor to explain the
phenomenon.
The next time the weather forecast predicts a severe frost,
forget the kettle and reach for the de-icer instead. |
| Richard
Angwin |
The
professor was stumped but promised to investigate the matter further.
To his surprise he found that, despite his best efforts, Mpemba
was correct.
While
the 'Mpemba Effect' has been proved, an explanation has been rather
more difficult to establish. There are at least four different theories
to explain the phenomenon.
- Evaporation - as the warmer water cools it may lose
a significant amount of its mass due to evaporation. The reduced
mass will make it easier for the warmer water to reach freezing.
According to some scientists evaporation is not enough to explain
the effect.
- Dissolved
gases - hot water can hold less dissolved gas than cold
water. So the initially warmer water will hold less dissolved
gas than the cooler water. This may encourage convection currents
within the water, thereby accelerating the cooling effect.
- Convection
- generally, the density of a liquid decreases with increasing
temperature. This usually makes the surface of a liquid cooler
the bottom. An initially warmer liquid, upon cooling to an average
temperature the same as that of the initial temperature of the
cooler water, will have a greater rate of cooling because of
the warmer surface layer. This complicated explanation is still
somewhat controversial.
- Surroundings
- in some cases the initially warmer water may cause a change
in the surrounding environment. For example, if the container
is sitting on a layer of frost, the warm water may cause the
frost to melt, thereby setting up a more efficient cooling system.
Unless
you are a serious ice cream addict the best advice is probably not
to take any notice of the Mpemba Effect.
But
the next time the weather forecast predicts a severe frost, forget
the kettle and reach for the de-icer instead.
by
Richard Angwin
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