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Related: About this forumWhy Hot Water Freezes Faster Than Cold—Physicists Solve the Mpemba Effect
Water may be one of the most abundant compounds on Earth, but it is also one of more mysterious. For example, like most liquids it becomes denser as it cools. But unlike them, it reaches a state of maximum density at 4°C and then becomes less dense before it freezes.
In solid form, it is less dense still, which is why standard ice floats on water. Thats one reason why life on Earth has flourished if ice were denser than water, lakes and oceans would freeze from the bottom up, almost certainly preventing the kind of chemistry that makes life possible.
Then there is the strange Mpemba effect, named after a Tanzanian student who discovered that a hot ice cream mix freezes faster than a cold mix in cookery classes in the early 1960s. (In fact, the effect has been noted by many scientists throughout history including Aristotle, Francis Bacon and René Descartes.)
The Mpemba effect is the observation that warm water freezes more quickly than cold water. The effect has been measured on many occasions with many explanations put forward. One idea is that warm containers make better thermal contact with a refrigerator and so conduct heat more efficiently. Hence the faster freezing. Another is that warm water evaporates rapidly and since this is an endothermic process, it cools the water making it freeze more quickly.
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dimbear
(6,271 posts)Probably due to the way convection cells happen to form in the water, IMHO.
xfundy
(5,105 posts)Somebody lied to me.
struggle4progress
(118,228 posts)Jim__
(14,063 posts)... freezes faster.
But as the liquid warms up, it forces the hydrogen bonds to stretch and the water molecules sit further apart. This allows the covalent molecules to shrink again and give up their energy. The important point is that this process in which the covalent bonds give up energy is equivalent to cooling.
In fact, the effect is additional to the conventional process of cooling. So warm water ought to cool faster than cold water, they say. And thats exactly what is observed in the Mpemba effect.
If the temperature of both the warmer water and the cooler water are close to freezing, I can see the warm water freezing faster. But, say the warm water is at 850C and the cool water is at 450C. Then doesn't the warm water first cool to 450C and then continue following the same process that the cool water began with? Is it the fact that when the warm water has cooled to 450C, that is an average, and the water that started out warmer continues cooling faster because of its mixture of water in both higher and lower energy states?