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The Mpemba Effect is a special
phenomenon where hot water freezes faster than cold
water. The discovery of this effect was made by a high
school student named Mpemba in Tanzania, Africa in 1969.
He noticed this phenomenon while making ice cream and
was curious enough to make note of it. His teachers did
not believe it was possible, and it took several years
until university professors finally accepted his
discovery.
Questions you may have are:
-
What factors cause water to freeze?
-
How can the Mpemba Effect be
explained?
-
What are the experimental conditions
necessary for this effect?
How water freezes
To explain how the Mpemba Effect works,
you first need to understand the factors involved in how
water freezes.
Obviously, temperature is a factor in
water freezing.
Definitions
The temperature of water in a container
is the average energy of its molecules. The heat of the
amount of water is defined as total amount of energy of
all its molecules. Thus the heat is dependent on how
much water is in the container and thus how many
molecules there are in the container.
Changing state
When you place a container of water in
the freezer compartment of a refrigerator, the water
will cool and finally freeze. The temperature of the
water goes down as the heat of the water (energy of the
water molecules) is reduced. When the temperature
reaches 32oF or 0oC, the water changes from a liquid to
a solid state.
Temperature stays until frozen
Note that temperature of the water drops
until it reaches the freezing point. Then it will stay
at 32oF or 0oC until the water solidifies. After the
water turns to ice, its temperature can become lower if
the air temperature is lower.
Much of the cooling is done by conduction.
Container
The container is is direct contact with some cold
material, such as the freezer shelf. The container gets
cold and also cools the water by conduction.
Metal is a good conductor, so a metal pail would speed
up cooling the water by conduction. On the other hand,
wood is a poor conductor of heat. A wooden pail would
require other heat transfer methods to cool the water.
Air
Cold air is in contact with both the container and the
water. The water transfers heat to the cooler air by
means of conduction, thus lowering the temperature of
the water.
Convection is the transfer of energy through the
movement of currents of a gas or liquid. You can see
this motion when heating a pot of water on a stove. You
can also feel the effect of it on a cold, windy day.
Different densities cause water convection
Since cold water is more dense than warm water, it will
sink to the bottom of the container, causing some
convection currents during the freezing process. When
the temperature of the water gets below 39oF or 4oC, it
becomes less dense and will float to the top until the
water finally freezes.
Air convection
There is also the effect of the movement of cold air.
Some freezers have small fans to move the cold air
around, so the water can be cooled by this air
convection. A windy day in the winter can cool things
much more than a still day at the same temperature.
Slowing freezing
In some situations when water is moving, it can actually
increase the time it takes to freeze as compared with
still water. For example, ducks often paddle around in a
pond in the winter to keep it from freezing over.
When a liquid evaporates, the higher energy molecules
leave the lower energy molecules behind, resulting in
lowering the temperature of the material. You can
experience that by spreading some water on your skin and
blowing across it to enhance evaporation. there is more
evaporation from hot water than from cold.
Warm water may radiate out some its energy, but the
amount of cooling caused by radiation is negligible.
Explanation of effect
Using knowledge of the factors involved in how water
freezes, scientists tried to come up with an explanation
of the Mpemba Effect. The factors that allow this effect
to take place are conduction, evaporation, convection
and dissolved gases. Since this is a complex effect,
these explanations are somewhat speculative.
One theory is that frost on a container can slow down
the cooling process.
If hot water is placed in the freezer in a small
container that is a good conductor of heat (or cold),
the warmth of the container can melt any frost that
collects on its surface. This includes the ice on the
bottom surface. When this ice refreezes, it creates a
good connection between the container and the surface,
allowing much better conduction of cold than a container
of cold water that has frost on its surface, including
its bottom. As a result, heat is drawn out of the warmer
container more rapidly than the one with cold water in
it.
Just as a layer of frost on the surface of a container
can slow down the conduction of heat from the water, a
layer of ice on the upper surface of the water can
insulate the water from the from the colder air
currents.
Since water becomes less dense between 37oF and 32oF
(3oC and 0oC), it will float to the top and then finally
freeze. This thin layer of ice will then act as an
insulator protecting the water below from freezing
rapidly and will slow down the freezing process of cold
water.
In the case of warm water, the convection currents will
cause that ice to melt, allowing the water to cool more
rapidly.
It is thought that evaporation is one factor that allows
warm water to freeze faster than cold water. There is
more evaporation from warm or hot water than from cold
water. Thus the evaporation not only carries off some of
the water, resulting in slightly less water to freeze in
the warm water container, but it also causes the
temperature of the warm container to drop due to heat
lost.
If the container is made of something like wood, which
is a poor conductor of heat (or cold), then a great part
of the cooling will be caused by evaporation instead of
conduction. This could be an important factor in
explaining how hot water freezes faster than cold. For
example, Mpemba used wooden buckets when he was making
his ice cream and noticed the phenomenon.
One more possible factor concerns that fact that water
always contains dissolved gases such as oxygen and
carbon dioxide. These impurities have the effect of
lowering its freezing point.
When water is heated, gases are driven out because their
solubility in water is less at higher temperatures.
Thus, when the hot water cools, it has less dissolved
gas than water which was not heated, so it has a higher
freezing point and freezes first.
Experimental conditions
Knowing the factors involved in freezing warm water, you
can attempt to duplicate the Mpemba Effect. In any good
experiment, you want to change only one variable and
keep everything else to the same. You must also be able
to determine when you achieved the outcome of the
experiment.
Factors that must remain the same are:
-
The temperature of the freezer
-
The amount of water in the container
-
The size, shape and material of the container
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Any type of air motion over the water
The only thing you vary is the initial temperature of
the water.
A big problem is determining when the water is frozen. Mpemba was making ice cream when he made his discovery.
He wasn't using a timer, but was observant enough to
notice the difference in freezing time.
You could use a thermometer or temperature probe in the
water to see when the temperature near the bottom levels
off at the freezing point. Then check often to determine
when the water in the container is frozen. Perhaps a
clear container would help.
The best way to really test the theory is to perform the
experiment at a number of different water temperatures.
In this way, you could find the which warmer water
temperature froze quicker than water at a cooler
temperature. There may be other water temperatures for
which the effect does not hold.
Of course, you could be lucky and pick the correct
temperature ranges the first time.
You can do the experiment using one or even several
containers that you place in a freezer compartment.
With the Mpemba Effect, warm water can freeze faster
than cold water, under certain conditions. Evaporation,
conduction, convection and dissolved gases are possible
reasons the effect works. You may have to try different
configurations to verify this effect. |
