What that whole essay seems to be saying is that the hot water freezing faster is an illusion, caused by other factors which act upon the water.
Apparently, quite a few things need to be controlled in order to test the hypothesis that hot water freezes faster. The water must be degassed (to control for the change in freezing point caused by solutes). The water must be prevented from evaporating, which causes other complications—hot water can only be prevented from evaporating by putting it in a container that does not expand, and preventing it from expanding increases the pressure, and increasing the pressure increases the freezing point. So the cold water would have to be subjected to the same pressure. The water must be cooled slowly, so as to avoid the supercooling effect. And the water should be stirred to maintain an even temperature—but the stirring will lower the freezing point—although this may not be necessary if the cooling is sufficiently slow to prevent supercooling.
I remember doing crystallization studies back in undergrad chemistry classes. We had to put crystals into a tiny glass tube, and put the tube into a heater that took forever to heat. Then we sat there, looking through a magnifier at the crystal until it started to melt, and record the temperature. As I recall, the temperature change had to be very slow to get an accurate reading.
I also recall from those chemistry classes that a pure substance has to spend some time at a particular internal temperature before it undergoes a phase change. That is because the substance must gain or lose internal kinetic energy (depending on the direction of the phase change) before it can undergo the transition. Once the energy is gained or lost, the phase change occurs fairly rapidly.
"But by the 20th century the phenomenon was only known as common folklore, until it was reintroduced to the scientific community in 1969 by Mpemba, a Tanzanian high school pupil. "
Something about this rings false to me. In 1965, a high school teacher of mine challenged me to do this same experiment at home, in private conversation with him. Later on that week or maybe a month later, he extended that same challenge to others in my class. So, somehow it was not "reintroduced to the scientific community" by someone who lived in Africa, but was apparently widely known in America.
By the way, no one who did the experiment found that hot water froze more quickly. The teacher laughingly explained that it success was more an effect of poor housekeeping -- that hot water melted ice/frost in the freezer, allowing the container more intimate contact with the freezer coils.
After reading all that, I’m now more convinced that it DOES have something to do with the lopsidedness of the water molecule and the easier crytalization of a more uniform molecular array. What other molecule has the unique property of water that it is expands to reduce density when frozen?
After reading all that, I’m now more convinced that it DOES have something to do with the lopsidedness of the water molecule and the easier crytalization of a more uniform molecular array. What other molecule has the unique property of water that it expands to reduce density when frozen?