Mpemba Effect: When Hot Water Freezes before Cold
by Ron Kurtus
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 of experimentation until the scientific community finally accepted his discovery.
Questions you may have include:
- What factors cause water to freeze?
- How can the Mpemba Effect be explained?
- What are the experimental conditions necessary for this effect?
This lesson will answer those questions. Useful tool: Units Conversion
How water freezes
To explain how the Mpemba Effect works, you first need to understand the factors involved in how water freezes.
Heat and temperature
Obviously, temperature is a factor in water freezing.
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.
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 thermal energy of the water molecules is reduced. After the temperature reaches the freezing point of 32oF or 0oC, the water starts to change 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.
The container is in 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.
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 denser 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.
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.
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.
Good conduction and good contact
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.
Convection and insulating surface
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 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.
Poor conduction and evaporation
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.
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
- 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.
Different starting temperatures
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 which warm 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.
(See Experiments with States of Matter: Mpemba Effect to set up the experiment.)
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.
Take note of the unusual
Resources and references
Can hot water freeze faster than cold water? - University of California, Dept of Physics
Experimental Details - James Brownridge; Binghamptom University (SUNY) Department of Physics
Mpemba Effect - Wikipedia encyclopedia
Knight, Charles A., "The Mpemba Effect: The Freezing Times of Hot and Cold Water", Letter in American Journal of Physics, Vol 64, May 1996, p524
Auerbach, David, "Supercooling and the Mpemba Effect: When Hot Water Freezes Faster Than Cold", American Journal of Physics, Vol 63, 882-885, (1995)
Scientific American article published in 1977. Volume 237, No. 3, pages 246-257
Dorsey, N.E., Am. Philos. Soc. 38, 247-328, (1948).
Dorsey, N.E., The Properties of Ordinary Water Substance, Reinhold, Scranton, PA., (1940).
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Mpemba Effect: When Hot Water Freezes before Cold