The Mpemba Effect
I was alerted to this tale of a scientific curiosity by The Times.
A Tanzanian student; Erasto Mpemba, found that hot water freezes quicker than cold water, contrary to what would be expected. It is now called the Mpemba effect.
I don’t find it surprising that no-one has fully explained the phenomenon, despite it apparently being known to such as Aristotle.
I think it does show though, that sometimes anybody can make a scientific discovery with the most basic of equipment. And in most cases, to be taken seriously by the establishment.
There are some curious phenomena out there in the real world.
One is that when water freezes it expands and thus ice always floats on ponds. If it didn’t you wouldn’t get any fish in water that could freeze.
And then there is the odd property of the speed of sound in air and water. In the former it is 343.2 metres/second and in water it is 4.3 times as fast at 1484 metres/second. Now I know my physics and when asked what the speed of sound in a bubbly mixture of air and water is, I did what I thought was obvious and said somewhere in between.
I was of course wrong, as surprisingly it is less than 50 metres/second. There’s an interactive display here.
I have used this phenomenon to mix oil and water. They do mix, if you get the parameters right.
The Anonymous Widower 
I know how it works.
Comment by John Wright | January 16, 2013 |
When you get down too it, I would argue there is no experiment showing a real mpemba effect with identical containers and pure water. As you rightly discuss, there are many ways experiments on this can fail – by not having pure water, or by not using identical containers, or having the cooled under identical conditions. I would argue that it is actually fairly simple to explain the effect once you look at J. Brownridge’s extremely careful experiments. He found that the effect is due to minute differences in the sample container walls. Even in containers that appear identical, they can have different nucleation sites, which allow initially hot water to freeze first, because it can freeze at a higher supercooled temperature than cold (-4 degrees instead of -10 degrees, for example). If identical containers could be used (or the same container used, and cooled under identical conditions) there is no reason to believe the hot water would freeze first. I have written a blog post on this:
https://moreisdifferent.wordpress.com/2013/04/20/does-hot-water-freeze-faster-than-cold/
Comment by MoreIsDifferent | September 22, 2015 |