Background Information Erasto Mpemba, a high school student in Africa, observed a phenomenon in 1963 that seemed impossible: a hot solution froze faster than a colder one. He asked his teacher how that could have happened. The teacher responded that his student must have been confused (Mpemba & Osborne, 1969).
Mpemba persisted. As it turns out, the student was not confused about his observation, which has since become known among scientists as the Mpemba effect and has been studied many times (Jeng, 2006). Still, a half-century later, scientists have yet to agree on an answer to the question.
It is something “that still has scientists baffled,” according to an article in Science Alert by Fiona MacDonald (2017). “It turns out that freezing water is a lot more complicated than you might think” (p. 2).Monwhea Jeng, who wrote about the Mpemba effect in a 2006 paper, said it “occurs when two bodies of water, identical in every way, except that one is at a higher temperature than the other, are exposed to identical subzero surroundings, and the initially hotter water freezes first.
” Jeng noted that it appears “theoretically impossible” because it violates the laws of thermodynamics, but has been observed in “numerous experiments” (p. 1). He also traced the phenomenon back to great philosophers, including Aristotle and Descartes (p. 3).However, not all of the experiments have reached the same result. In an experiment, a college student conducted three trials (Whiteman, 2013.
) He froze four samples of water, each measuring 50 mL. The samples had different starting temperatures, 57 degrees, 95 degrees, 130 degrees and 180 degrees fahrenheit. In every trial, the water at the coldest temperature froze first, contradicting the Mpemba effect (Whiteman, 2013).In “Cool?,” the paper that Mpemba published in 1969 with Professor Denis Osborne of the University College Dar es Salaam in Tanzania, the student detailed how he first noticed that hot liquid freezes first when he put hot milk into the freezer to make ice cream at about the same time another student put milk in that was not boiled.
He repeated the experiment with water, and then with hot milk, with the same result both times. The hot liquid froze first. Osborne, who also tested the phenomenon in a lab, concluded that the explanation for the faster cooling of the hotter liquid is convection (Mpemba & Osborne, 1969).”The rate of cooling depends on the surface temperature of the liquid and not its mean body temperature,” the professor wrote.
“Convection within the liquid maintains a ‘hot top’ (presumably while about 4 degrees C) and the rate of loss of heat for an initially hot system can be greater than for an initially cooler system, even when they have cooled to the same mean body temperature” (Mpemba & Osborne, 1969, p. 3). In addition to convection, Jeng offered four other explanations for the faster cooling of the initially warmer liquid, including: evaporation, dissolved gases, the surroundings, and supercooling (Jeng, 1998). Jeng said evaporation is not the sole reason why the Mpemba effect can take place, but may be a contributor to the phenomenon.
He wrote: “The explanation of the effect is that as the hot water cools, it loses mass to evaporation. With less mass, the liquid has to lose less heat to cool, and so it cools faster” (Jeng, 1998). Another explanation is that dissolved gas is removed from the hotter water so that it changes the freezing point of the water. However, Jeng asks “whether this can significantly affect the properties of water in a way that explains the Mpemba effect” (Jeng, 1998).
Yet another explanation is that the initially hot water changes the environment, suggesting that the freezer is a variable in the experiment. For example, the hot water may cause the container to melt the frost beneath it so it has more contact with the freezer and that causes the water to cool faster. Jeng, however, dismissed this explanation for most of the experiments. (Jeng, 1998).Supercooling is when water freezes at a temperature below the normal temperature of solidification. In order to freeze, water needs an imperfection or irregularity as a nucleation site. Water may need to reach sub-zero temperatures until a nucleation site forms.
Separate studies have shown that water with a higher starting temperature will not supercool as much, meaning the colder water will have to reach a lower temperature, taking longer (Jeng, 1998, p. 6-7). Supercooling also was considered by David Auerbach, who concluded that the Mpemba studies had shown hot water is supercooling before freezing, and that the cold water supercools to a lower local temperature than the hot before freezing (Auerbach, 1995). Jeng concluded that “while quite a few experiments have replicated” the Mpemba effect, “there has been no consensus” on what causes it. He also writes that “it seems likely that there is no one mechanism that explains the Mpemba effect for all circumstances, but that different mechanisms are important under different conditions” (Jeng, 1998, p. 2).
Therefore, an experiment was designed to replicate the Mpemba effect, and the results that Mpemba achieved, in three trials that tested whether hot water of the identical amount measured by volume and weight in identical container shapes, types, and materials will cool faster than cold water.