There is this puzzling thing that is called Mpemba effect: paradoxically, warm (35°C) water freezes faster than cold (5°C) water. As a physisist, I've been asked about it several times already. And I have no definite answer to that, apart from the standard: "there are many things that can influence it".
So, does anyone knows about the status or progress on that effect? Any recent reviews, publications or other references?
One boring Monday morning in the lab a group of us did the experiment, and to our surprise we found that the hot water (in sealed containers) did freeze faster.
On closer examination we discovered that the shelves in our freezer were covered in frost, like I imagine most freezers, and the hot water was melting the frost and creating a good thermal contact between the beaker of water and the shelf. That turned out to be why the hot water froze faster. When we thoroughly cleaned the freezer shelf the effect went away and the hot water took longer to freeze.
I think the rumours about hot water freezing faster illustrate the dangers of improperly controlled experiments. As Ron mentions, evaporation could also be a factor and it would be easy for a home experimenter to get the wrong conclusion. Add to that the fact we'd secretly all be delighted if we could prove hot water really does freeze faster, and you can see how the rumour has spread.
My paper also participated in RSC competition Mpemba effect but because there were too many entries (over 22000) the first screening was made by a team of journalists.I give you the abstract: Water molecules are dipoles positioned and oriented joined by hydrogen bonds. When water is heated this structure collapses (increasing entropy). After the water is recooled to a lower temperature the structure is not reconstructed immediately but needed some time. Sometimes time is not enough inside a freezer because the cooling process is too fast. Entropy reduction curves function of temperature S=f(T) appear retardation (lagging) relative to entropy growth curves. The water after was heated and recooled at the initial temperature, has more entropy than before it was heated. This means that molecules have now the same kinetic energy,but thermal motion before heating was more oriented by the structure mentioned above. Recooling random collisions are more possible leading to faster temperature’s reduction.Look:New Explanation
Warmer water will tend to bond faster but in a more disorganized fashion. Molecules at a temperature closer to the 'freezing' point will take more time to bond but the packing fraction will be more efficient. This is the same philosophy behind crystal manufacturing. The Bottom line is that under the same exact conditions warmer water will make ice faster than colder water, but the ice is not the same. the ice made with warmer water will melt faster. I know there are those who doubt me but ask the people at Beverly Ice, they will tell you the same thing. so will your intel crystal grower, the good stuff takes time. Oh and I double checked last night using sealed bags so frost would not be a consideration.
The explanation is that hot water evaporates, leaving less water to freeze (but see John Rennie's answer for a contact effect which is probably the biggest factor for most of the reports of the effect. Aside from John's effect...) this is the only significant difference, there is no other, despite what you sometimes read. The evaporation effect is not even that big, if you add 10 degrees of temperature to the water, the difference in evaporation is negligible and the hot water will freeze by an amount of time which is longer by exactly the time it takes to cool down the extra 10 degrees.
All experiments that claim that a closed container of hot water freezes faster are unreliable (but see John Rennie's answer). But nobody freezes in closed containers for obvious reasons. If you freeze trays covered with plastic-wrap, hot water freezes slower. There is no "Mpemba effect", and I feel it is not nice to attach such a thing to an obscure student who was reporting something true from experience.
