I should know better. After all these years of marriage you think I would have learned by now - that right or wrong, there are long-term ramifications for attempting to contradict the Mrs. by asserting any measure of intellectual superiority.
My wife, bless her heart, is a firm believer in old wives tales. She cuts the ends off of cucumbers and rubs them vigorously against the rest of the uncut portion prior to slicing it "to draw out the flavor". She won't go swimming after she eats without waiting for an hour, and believes that if you swallow your gum it will stay in your system for 7 years. (How does it know?). There are others, all harmless enough, and I attribute her beliefs to her staunch Irish-Catholic upbringing and small, mid-west town sensibility. They are an inexorable part of her charm.
I, on the other hand, dismiss such folklore completely. As an engineer, my reality exists on facts and data and until last week, I never challenged my wife on the validity of her wives tales. Here is where it begins to get ugly.
"Why are you filling the ice cube trays with steaming hot water before putting them in the freezer?" I asked.
"Because it will freeze faster than cold water." She replied.
I shook my head in pitiful disbelief.
"What? Everybody knows that!" She exclaimed.
I gently explained to her that it was simply not true. In fact, it was impossible. "Look", I said. "Let's say you have a tray of initially cooler water whose temperature is at 30° C and it takes 10 minutes to freeze, and an identical tray of warmer water equal in volume that starts out at 70° C. Now the initially warmer water has to spend some time cooling to get to get down to 30° C, and after that, it's going to take 10 more minutes to freeze. So, since the initially warmer water has to do everything that the initially cooler water has to do, plus a little more, it will take at least a little longer, right?"
"I suppose that makes sense." She said thoughtfully.
I went to the computer to find a reliable reference and prove my point. It turns out that she was right! Well, at least she may be right. There is at least some validity to this old wives tale. It is a phenomenon known as the "Mpemba effect".
What I implicitly assumed in my proof to my wife, is that the water is characterized solely by a single number -- the average temperature. But if other factors besides the average temperature are important, then when the initially warmer water has cooled to an average temperature of 30° C, it may look very different than the initially cooler water (at a uniform 30° C) did at the start. The water, in fact, may have changed when it cooled down from a uniform 70° C to an average 30° C. It could have less mass as a result of evaporation, less dissolved gas, or convection currents producing a non-uniform temperature distribution. The change could occur as a result of the effects of supercooling. Or it could have changed the environment around the container in the freezer. All of these changes are conceivably important. [1., 2., 3.]
Apparently, the fact that hot water freezes faster than cold water has been known for many centuries. The earliest reference to this phenomenon dates back to Aristotle in 300 B.C. The phenomenon was later discussed in the medieval era, as European physicists struggled to come up with a theory of heat. 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 student. Since then, numerous experiments have confirmed the existence of the "Mpemba effect", but have not settled on any single explanation.
The genesis of Mpemba's experiments began in 1963 when he was making ice cream at school, by mixing boiling milk with sugar. He was supposed to wait for the milk to cool before placing it the refrigerator, but in a rush to get scarce refrigerator space, put his milk in without cooling it. To his surprise, he found that his hot milk froze into ice cream before that of other students. He asked his physics teacher for an explanation, but was told that he must have been confused, since his observation was impossible.
Mpemba believed his teacher at the time. Later that year he met a friend of his who made and sold ice cream in his town. His friend told Mpemba that when making ice cream, he put the hot liquids in the refrigerator to make them freeze faster. Mpemba found that other ice cream sellers in town had the same practice.
Later, when in high school, Mpemba learned
A Dr. Osborne, a professor of physics, had visited Mpemba's high school. The student asked him to explain why hot water would freeze before cold water. Dr .Osborne said that he could not think of any explanation, but would try the experiment later. When back in his laboratory, he asked a young technician to test Mpemba's claim. The technician later reported that the hot water froze first, and said "But we'll keep on repeating the experiment until we get the right result." However, repeated tests gave the same result, and in 1969 Mpemba and Osborne published their results.[1.]
When I contritely revealed what I had discovered to my wife, she responded with only a satisfied smile. I know that smile. It's one she will hold in reserve for the next time I try to assert my supposed intellectual superiority.
[1.] Mpemba and Osborne, "Cool", Physics Education vol. 4, pgs 172--5 (1969)
[2.]
[3.] B. Wojciechowski, "Freezing of Aqueous Solutions Containing Gases", Cryst. Res. Technol., vol. 23, #7, pgs 843--8 (1988)
