Mabemba's Water Theory

While the effect appears impossible at first sight, it has been seen in numerous experiments, was reported on by Aristotle, Francis Bacon, and Descartes, and has been well-known as folklore around the world. It has a rich and fascinating history, which culminates in the dramatic story of the secondary school student, Erasto Mpemba, who reintroduced the effect to the twentieth-century scientific community. The phenomenon, while simple to describe, is deceptively complex, and illustrates numerous important issues about the scientific method: the role of skepticism in scientific inquiry, the influence of theory on experiment and observation, the need for precision in the statement of a scientific hypothesis, and the nature of falsifiability. We survey proposed theoretical mechanisms for the Mpemba effect and the results of modern experiments on the phenomenon.

Studies of the observation that hot water pipes are more likely to burst than cold water pipes are also described.

Firstly I will relate a story about a surprising experiment. The experiment is based on an observation made on a number of occasions that appears to go against common sense. The observation is that if approximately equal amounts of a hot and a cold liquid are placed together in a freezer, then the hot liquid freezes first. This seems to me to be a prime example of what science educators call a discrepant event (cognitive dissonance/cognitive conflict). A discrepant event is a happening contrary to our current beliefs. Discrepant events are said to be useful in enabling learners to reconstruct concepts that have been imperfectly understood. The literature on discrepant events is comparatively small with the following being the main easily accessible references (Fensham &Kass, 1988; Hand, 1988; Thompson, 1989).

One of many unsolved mysteries of science, It will remain a mystery for me to achieve satisfactory and logical results, the physics teacher advised me to repeat Experiments to prove the results and to make sure it is real. So basically what I'm trying to do is to explain Mpemba theory and find out is it the truth that the hot water freezes faster than the cold water? I suggest that the origin of the Mpemba effect (the freezing of hot water before cold) is due to freezing-point depression by solutes, either gaseous or solid, whose solubility decreases with increasing temperature so that they are removed when water is heated. The solutes are concentrated ahead of the freezing front by zone-refining in water that has not been heated, reducing the temperature of the freezing front, and thereby reducing the temperature gradient and heat flux, slowing the progress of the freezing front.

Method:

Apparatus used:

• Small freezer with an internal temperature: to freeze the water of-19.1° to -18.8°C

• Cylindrical aluminum calorimetric vessels measuring 65 mm in height by 48 mm in diameter

• Electric kettle: to boil the water

• Deionized water: water free from all charged atoms or molecules, used mainly in the manufacture of water-based cleaning chemicals.

• Digital data logger: To record data over time.

• Temperature probes: To measure the initial temperature.

• Paper towels:

• Cling film: to cover the freezing water

• Masking tape: to cover the area on which unwanted substance not needed

• Mains power supply: to power the small freezer and Electric Kettle.

To obtain varying initial temperatures, cold deionized water was made up to different depths in six aluminum vessels(bare and open; bare and sealed on the top with cling film; insulated and open; or some combination of these). Boiled water from a kettle was used to top the water up so that the total volume of water in each vessel was 100 ml. Digital temperature probes were secured with masking tape so that the 8 mm-long head of each probe was fully submerged at the surface of the water and these were connected to a data logger that sampled the temperature of each probe at 10 intervals. Each vessel was placed on an insulating layer of the folded paper towel to minimize conductive heat loss through the layer of frost on the shelf of the freezer. A schematic of the experimental setup is presented in figure 1.

A number of mechanisms have been hypothesized to explain the Mpemba Effect. Monwhea Jeng [2]and Marek Balazovic and Boris Tomazik [3] have written an excellent overview of the subject summarising these hypotheses. One is that the initially hotter vessel melts the frost layer on which it sits more completely than the colder vessel does; when this refreezes it creates a better thermal contact that draws heat away more rapidly.

By placing the vessel on insulating layers of folded paper towel the possibility of the efficacy of this hypothesis was immediately eliminated.

Boiling the water first also reduced the presence of dissolved gases, which had also been claimed to contribute to the effect.

Supercoiling, when a liquid remains fluid below its freezing point before spontaneously becoming solid, has also been proposed as an explanation. James D Brownridge states:‘Hot water will freeze before cooler water only when the cooler water supercoils, and then, only if the nucleation temperature of the cooler water is several degrees lower than that of the hot water.

Heating water may lower, raise or not change the spontaneous freezing temperature.’ [4]While this may be the case in some circumstances, it is not a satisfying explanation for the following reasons. Firstly, supercoiling is temperamental.

Graph of temperatures for two insulated vessels; the water in the vessel represented by the blue line starts only 4.45 °C hotter than that of the red line but begins to freeze in 15.5% less time.

Graph of time at commencement of freezing against initial temperature. Red triangles: bare, cling film-covered vessels. Blue squares: insulated, open vessels; this graph is similar to that presented in Mpembaand Osborne’s 1969 paper.

The phenomenon was not observed to occur to temperatures below approximately -1 °C if it occurred at all. Impurities in the water, imperfections in the surface of the inside faces of the vessel and even the very presence of the head of the temperature probe tended consistently to cause nucleation and freezing at or very close to 0 °C.

Efforts were made to encourage supercoiling(the use of deionized water and in some experiments only submerging the tip of the temperature probe), but supercoiling was still difficult to achieve. Secondly, Mpemba first observed the effect in ice cream, which is most unlikely to supercool; ideally, we would like a general explanation that also explainsMpemba’s original observations. Thirdly, the Mpemba Effect was observed a number of times without supercoiling, as figure 2 shows.

Evaporation and convection have also been proposed separately (by Mpemba in his original paper and by Jeng, Balazovic, and Tomazik). This investigation finds evidence to suggest that the effect is caused by a combination of both of these mechanisms.

Mpemba’s original observation with ice cream likely used ceramic (insulating) vessels and Osborne’s experiments used Pyrex vessels, so it was thought that the Mpemba Effect might be related to the insulation of the vessels in which the water is held. Aluminum vessels were chosen because they allowed easy adaptation of the vessels for different experiments. Leaving the vessels bare and adding a cling film covering to the top to suppress evaporation allowed the cooling behavior of the water when radiation was the main mode of heat loss to be investigated. If a wrapping of paper towel insulation were added around the side and base of the vessel, surface radiation and evaporation would be the main modes of heat loss. This allowed the effect of evaporation to be separated and analyzed. As predicted, theMpemba Effect only occurred in insulated containers, suggesting that it had to do with surface cooling effects. Figure 3 compares the graph of time at commencement of freezing against initial temperature for insulated vessels(blue squares) and cling film-covered vessels (red triangles). The clingfilm-covered vessels behave in an intuitive way: the higher the initial temperature, the longer it takes for the water to begin to freeze. With insulated open vessels above a certain temperature, any further increases in temperature cause a decrease in the freezing time. This suggests that evaporation was a contributing factor to the Mpemba Effect. More data over the full range of temperatures from freezing to boiling would be useful to further investigate the shape of this graph. I would like to point out that time is an essential element of this experiment and the basis for doing so, and all these trials depend on time.