主题：【未解之迷】问一个普通的问题:温水为什么比冷水结冰快？ -- 不爱吱声

Mpemba当年是一个坦桑尼亚的高中生。下面是一个简短的说明。更长的说明见

外链出处

(Written Nov, 1998 by Monwhea Jeng (Momo),

Department of Physics, University of California)

简短地说，那种认为因为热水要结冰必须首先变成冷水，所以热水结冰必须比冷水慢的理由并不确切，因为一杯水的状态并不是可以只用温度一个量就可以描述出来的。可以有这种可能，从热水由降温而成的冷水，虽然化了点时间降到了原来那杯冷水的同样温度，但是它的其它特性那时却变得和那杯冷水不一样，这种特性使得它在接下来的冷却过程中冷得更快。

引起这个效应的原因是复杂的，也不是热水永远比冷水早结冰，还有“开始结冰”和“完全冻住”也是不同的概念。热水比冷水早结冰需要特定的条件，下面的说明建议你热水的温度要高，制冷主要由蒸发造成，你就有可能使热水比冷水早结冰。详细的说明必须看我上面给出的链接。

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Hot Water Freezes Faster than Cold!　　　　　　 updated 11-May-1992 by SIC

-----------------------------------　　　　　　 original by Richard M. Mathews

--You put two pails of water outside on a freezing day.　One has hot

water (95 degrees C) and the other has an equal amount of colder water (50

degrees C).　Which freezes first?　The hot water freezes first!　Why?　

--It is commonly argued that the hot water will take some time to

reach the initial temperature of the cold water, and then follow the same

cooling curve.　So it seems at first glance difficult to believe that the

hot water freezes first.　The answer lies mostly in evaporation. The effect

is definitely real and can be duplicated in your own kitchen.

--Every "proof" that hot water can't freeze faster assumes that the

state of the water can be described by a single number.　Remember that

temperature is a function of position.　There are also other factors

besides temperature, such as motion of the water, gas content, etc. With

these multiple parameters, any argument based on the hot water having to

pass through the initial state of the cold water before reaching the

freezing point will fall apart.　The most important factor is evaporation.

--The cooling of pails without lids is partly Newtonian and partly by

evaporation of the contents.　The proportions depend on the walls and on

temperature.　At sufficiently high temperatures evaporation is more

important.　If equal masses of water are taken at two starting

temperatures, more rapid evaporation from the hotter one may diminish its

mass enough to compensate for the greater temperature range it must cover

to reach freezing.　The mass lost when cooling is by evaporation is not

negligible. In one experiment, water cooling from 100C lost 16% of its mass

by 0C, and lost a further 12% on freezing, for a total loss of 26%.

--The cooling effect of evaporation is twofold.　First, mass is

carried off so that less needs to be cooled from then on.　Also,

evaporation carries off the hottest molecules, lowering considerably the

average kinetic energy of the molecules remaining. This is why "blowing on

your soup" cools it.　It encourages evaporation by removing the water vapor

above the soup.

--Thus experiment and theory agree that hot water freezes faster than

cold for sufficiently high starting temperatures, if the cooling is by

evaporation.　Cooling in a wooden pail or barrel is mostly by evaporation.

In fact, a wooden bucket of water starting at 100C would finish freezing in

90% of the time taken by an equal volume starting at room temperature. The

folklore on this matter may well have started a century or more ago when

wooden pails were usual.　Considerable heat is transferred through the

sides of metal pails, and evaporation no longer dominates the cooling, so

the belief is unlikely to have started from correct observations after

metal pails became common.

　　　 "Hot water freezes faster than cold water.　Why does it do so?",

　　　　Jearl Walker in The Amateur Scientist, Scientific American,

　　　　Vol. 237, No. 3, pp 246-257; September, 1977.

　　　 "The Freezing of Hot and Cold Water", G.S. Kell in American

　　　　Journal of Physics, Vol. 37, No. 5, pp 564-565; May, 1969.