Could it be related to the fact that ice is less dense than liquid water? That seems contradictory to intuition as well. I would think since hot water is less dense, its molecules are more spread out and so they need to move less to form ice. That's my immediate naive idea, I don't have time to actually think about it. Don't ask me to go any deeper or use math on that either.
Is this a joke?
Not as far as I'm aware, but then it was only reported by the BBC at 1pm today as coming from Imperial College, and The Royal Institute of Chemistry.
Mpemba placed cold water in a refrigerator and measured the time to freeze. Then placed hot water in the fridge and saw a shorter time to freeze...
What is the big mystery? When putting a heat source in a fridge the thermostat kicks on the compressor and actively cools down the interior.
What would be a mystery is if he had unplugged the fridge after the compressor shut off (so the fridge interior was at a known temperature), and THEN put the water in. If the hot water freezes faster without the aid of the compressor, then there is something to look at.
Or, if you set out two trays of hot and cold water outside on a windless, subzero day, in the shade...
Has anyone actually done the experiment correctly?
Really, I thought this was already solved. Hot water has less dissolved gases than cold water, so it has a higher freezing point.
Well at face value it sounds like a stupid contest and the link to the contest stie is broken...
That has little effect on the energy requirements, so little effect on the time.
Works for me, however I can't be accountable for american sites.
Why is it a stupid contest? Is there already an accepted explanation for the effect? I had heard of the effect a long time ago, but I always assumed there must have been a known explanation but was probably over most people's heads.
Looks real, its on Wikipedia! :-)
The existence of the effect outside of poorly set up (or purposely set up?) experiments is dubious at best and the fact that it was "discovered" by an Indian high school student in a cooking class implies it may be in line with other dubious or overblown "discoveries" by Indian schoolchildren we've seen floating around (in fairness, not all are Indian, they just seem overrepresented). The've become somewhat of a meme.
Tanzanian. Different skin color and different continent.
Eek, sorry. Not sure how I got that wrong.
excerpted from wikipedia on the Mpemba effect origin:
The effect is named after Tanzanian Erasto Mpemba. He first encountered the phenomenon in 1963 in Form 3 of Magamba Secondary School, Tanganyika when freezing ice cream mix that was hot in cookery classes and noticing that they froze before cold mixes. After passing his O-level examinations, he became a student at Mkwawa Secondary (formerly High) School, Iringa, Tanzania. The headmaster invited Dr. Denis G. Osborne from the University College in Dar Es Salaam to give a lecture on physics. After the lecture, Erasto Mpemba asked him the question "If you take two similar containers with equal volumes of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and put them into a freezer, the one that started at 100 °C (212 °F) freezes first. Why?" only to be ridiculed by his classmates and teacher. After initial consternation, Dr. Osborne experimented on the issue back at his workplace and confirmed Mpemba's finding. They published the results together in 1968.
 ^ Mpemba, Erasto B.; Osborne, Denis G. (1969). "Cool?". Physics Education (Institute of Physics) 4 (3): 172–175. Bibcode 1969PhyEd...4..172M. DOI:10.1088/0031-9120/4/3/312.
Today's lunch time news was the first I had ever heard of the phenomenon.
My immediate thoughts are:
A given liquid has a higher heat content at higher temperature so to reach freezing more quickly must require a greater rate of cooling and heat transfer for some reason.
Certainly the initial rate of cooling will be greater due to Newton's law of cooling.
However at some point the temperature of the originally hotter liquid must match or dip below that of the other.
So by the same reasoning it should cool more slowly from this point on.
This is an old "paradox" whose explanation is that the hot container will melt and sink into the layer of ice on the bottom of the icebox thus providing less thermal resistance for the remainder of the freezing process. Of course this only works on old, non "frost free" freezers.
I believe that the hermes2012 thing is a scam to capture personal information.
It seems quite simple in my mind,
The hot water has a more aggressive internal action, which will reflect a greater distribution of heat as it declines in temperature. The distribution of heat at a faster rate will reflect less heat overall throughout the liquid, this means a cooler central area.
When the two volumes are at an equal temperature, the hot has dispersed more heat as it declined and still has a greater internal kinetic action taking place, this continues to more evenly move heat to the boundary layer points of heat transfer, thus that center volume has a lower temperature and becomes solid before the cold container.
This is my thought process and no prior thinking, other than hearing the statement and had always discounted it's validity, the thread put my mind to work.
Assuming that this is a real effect, then is it only valid for water? If so, then the explanation can only apply to water.
Deadline today. I just did submit something and I wonder if own thoughts about the results of my little experiment can be posted here.
Separate names with a comma.