The Mpemba effect

CAN HOT WATER FREEZE FASTER THAN COLD WATER?

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Scientists have known for generations that hot water can sometimes freeze faster than cold water, an effect known as the Mpemba effect. The Mpemba effect is that boiling water can, under certain circumstances, not only freeze but do so faster than colder water. This phenomenon has been reported as working as far back as ancient Greece, even though it seems contradictory to the laws of Thermodynamics. In 1969, a scientist named Mpemba did experiments that proved that the effect is real. However, that left scientists with more questions than answers.

The Mpemba effect

Lets try to understand what we mean by Mpemba effect. We start with two water containers of identical shape and which holds identical amount of water. The only difference between the two is that water in one container is at a higher temperature than the other. Now we cool both the containers using the exact same cooling process for each container. Under some conditions the warmer water freezes first. If that occurs, you have seen the Mpemba effect. Of course, the warmer water will not freeze before the colder water for all conditions. If the hot water is at 99.9°C and the cold water at 0.01°C, then clearly under those circumstances the cold water will freeze first. But under some conditions the warmer water freezes first.

The effect is named after Erasto Mpemba who first encountered the phenomenon in 1963 when he froze ice cream mix that was hot in cookery classes and noticed that it froze before the cold mix. He asked his teacher, Dr. Osborne this question - "If you take two similar containers with equal volumes of water, one at 35°C and the other at 100°C, and put them into a freezer, the one that started at 100°C freezes first. Why?" only to be ridiculed by his classmates and teacher. After initial consternation, Dr. Osborne experimented on this issue back at his workplace and confirmed Mpemba's finding. The results were published in 1969. But there are no reliable sources that indicate exactly how to demonstrate the effect and under exactly what conditions it occurs.

Experiments on Mpemba effect

Some mechanisms suggested to explain the Mpemba effect are:

• Convention: When water cools it eventually develops convention currents and a non-uniform temperature distribution. Mostly, density decreases with increasing temperature, and so the surface of the water will be warmer than the top which is called hot top. If water loses heat primarily through the surface, then water with a hot top will lose heat faster than we would expect based on its average temperature. When the warmer water cools to an average temperature same as the initial temperature of the cooler water, it will have a hot top. Thus its rate of cooling will be faster than the rate of cooling of the cooler water at the same average temperature. Although experiments have seen the hot top and related convention currents, it is unknown whether convention can by itself explain Mpemba effect.
• Evaporation:  As the warmer water cools to the initial temperature of the cooler water, it may lose significant amount of water due to evaporation. Evaporation is endothermic, meaning the water mass is cooled by vapour carrying away the heat. The reduced mass will make it easier for the water to cool and freeze. Now the warmer water will cool before the cooler water, but it will make less ice. Theoretical calculations have shown that evaporation can explain the Mpemba effect, if water is assumed to lose heat solely through evaporation. But evaporation cannot explain experiments that were done in closed containers, where no heat was lost due to evaporation.
• Dissolved Gases:  Hot water can hold less dissolved gas than cold water. So it has speculated that it changes the properties of water in some way. Perhaps this makes it easier to develop convention currents helping it cool, or decreasing the amount of heat required to freeze unit of water, or changing the boiling point.
• Frost : Frost has insulating effects. The cooler water will tend to freeze from the top, reducing heat loss by radiation or air convention while the warmer water will tend to freeze from the bottom and sides due to water convention. This is disputed as there are experiments that account for this factor.
• Supercooling: Supercooling occurs when water doesn't freeze at 0°C, but at some temperature lower than that. One experiment found that the warmer water supercooled less than the cooler one. This means that warmer warmer water would freeze first because it would freeze at a higher temperature than the cooler water. That doesn't explain the Mpemba effect, because it is still unknown why warmer water supercools less than cooler water.

Ice cubes

Many such solutions have been suggested as reasons for the Mpemba effect, but none have been agreed upon by scientists and most contradictory explanations were obtained by very different experiments with different controls. Perhaps some day after more study scientists will understand this, but currently the results are inconclusive at best.