Sunday, November 3, 2013

Insight on the Concept of Mpemba Effect

Mpemba Effect

The Mpemba effect is the assertion that warmer water can freeze faster than cold water and is so named after a Tanzanian student, Erasto Mpemba from Africa. His first encounter with this phenomenon started in 1963 in Form 3 of Magamba Secondary School while freezing ice cream mix which was hot initially and his observation that it froze before the cold mix. On upgrading from his O level examinations, he went on to be a student of Mkwawa Secondary School in Iringa, Tanzania where the headmaster invited Dr. Denis G. Osborne from the University College, to conduct lecture on physics after which, Mpemba put forward a question asking why, when two similar containers with equal volume of water, one at a temperature of 35 degrees Cent; and the other at 100 degrees Cent; when placed in the freezer, the one with 100 degrees Cent; freeze first.

For this, Erasto Mpemba was ridiculed by his classmates as well as his teachers, but after initial confusion, Dr. Osborne experimented back at his workplace and confirmed Mpemba’s finding and the results were published in 1969. To understand the factors involved in how the water freezes will help to explain the Mpemba Effect. Here the temperature is the factor in water freezing and the temperature of water in the container is the average energy of its molecules where the heat of the water is defined as total amount of energy of its molecules. The heat depends on the content of the water and how many molecules are present in the container.
The change takes place when a container of water is placed in the refrigerator, in the freezer compartment, the water first cools and then freezes. When the temperature of the water drops down, the heat of the water – energy of the water molecules reduces and at this point, when the temperature reaches 0 degrees cent; the water changes from the liquid to a solid state. A point to be noted here is that the temperature of the water keeps dropping till it reaches freezing point and stays at 0 degrees cent; until the water becomes solid. When the water has turned into ice, its temperature can tend to be lower if the air temperature is lower. Most of the cooling is done through conduction, where the container is in direct contact with cold material namely the freezer shelf and gets cold thereby cooling the contents by conduction. Since metal is a good conductor, a metal container would quicken the cooling of water by conduction while wood is a poor conductor of heat.

Air which is in contact with the container as well as the water transfers heat to the cooler air through conduction which helps in lowering the temperature of the water. Convection is the transfer of energy through movement of currents of liquid which can be seen while heating a vessel of water on a stove, the effects which can also be seen on a cold windy day. Cold water being denser than warm water, will sink to the bottom of the vessel causing convection currents during freezing process and when the temperature reaches below 4 degrees cent; it tends to get less dense and float to the top till it finally freezes. Air convection also helps to move the cold air around so that the water gets cooled through air convention.

Evaporation is also another factor which helps the warm water to freeze quicker than cold water since the evaporation from hot or warm water is more, than in cold water and the during the evaporation process it carries some of the water resulting in less water to freeze in a particular container though it causes the temperature to drop due to heat loss in the warm container. Dissolved gases are another possible factor which has the effect of lowering the freezing point and water always has these impurities in the form of oxygen and carbon dioxide. As the water gets heated, the gases escape because they are less soluble in water at higher temperature and when hot water cools with less dissolved gas, it has a higher freezing point and freeze fast than the water which is not heated.

With the knowledge on the factors that are involved in the freezing process, it has been concluded that the Mpemba Effect takes place based on conduction, convection, evaporation, and dissolved gases. Another theory is that, frost can slow down the cooling process and if hot water is kept in the freezer in a container which is a good conductor of cold or heat, the heat of the container can melt away the frost that is collected on its surface including the ice at the bottom which when refreezes creates a good connection between the container and the surface. This provides a good conduction of cold than a container with cold water having frost on its surface as well as at the bottom, resulting in heat being drawn out from the warm container more quickly than with cold water in it. As the layer of frost at the surface can slow down the process of conduction of heat from the water, a layer on the surface on the upper side of the water can insulate water from colder air currents.

1 comment:

  1. This was very informative!
    I always thought cold water freezes faster! :)


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