by Ron Kurtus (revised 30 October 2002)
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 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. There is a mini-quiz near the end of the lesson. 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. 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. Conduction
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. 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
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. 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. Evaporation
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. Radiation
Warm water may radiate out some its energy, but the amount of...