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
有经验的老师傅告诉徒弟,冬天洗车最好用冷水而不用温水,否则温水一沾到车厢便会马上结冰。
奇怪,难道温水比冷水结冰快?徒弟问师傅:“为什么?”
师傅们无言以对。他们只知道,温水就是比冷水结冰快。
其实,师傅们解释不了这个奇怪的自然现象是非常正常的,因为迄今为止,连科学家也没有搞清楚:为什么冬天温水比冷水冻得快?当今世界上还没有人能够破解这个看似稀松平常的自然之谜。
据说,古希腊人曾发现了这个有意思的自然现象,但他们没有找到答案。
1969年,一名坦桑尼亚高中生艾拉斯托.穆宾巴正式向全世界提出这个问题?D?D?D为什么冬天温水比冷水冻得快?从那以后,这个问题才被全世界科学家所关注。
据说,1969年盛夏,艾拉斯托.穆宾巴想亲手制作冰激凌,他把一杯由牛奶和糖水等物质相混合的、还没有放凉的温热液体放进了冰箱冷冻室,结果他惊讶地发现,这次液体结晶得比以往任何一次都快,他很快就吃到了自己亲手制作的冰激凌。这个有趣的发现激发他深入研究的欲望。从那以后,艾拉斯托.穆宾巴相继做了很多温水冷冻实验,写了很多篇研究报告。由于艾拉斯托.穆宾巴的突出贡献,这个神奇的自然现象现在被科学界称为“穆宾巴效应”。
现在,在许多解释中最为普遍的理论为:温差理论,即冬天温水比冷水冻得快,是因为温水与周围环境之间的温差大于冷水与周围环境之间的温差,温差大温水中水分子的能量会很快散发到周围环境中。当然,这个理论仍然遭到许多科学家的质疑。因为按照这个理论,冷水与周围环境之间的温差小,冷水分子能量失去较慢,那么出现的问题是,温水终究要变成冷水,它变成冷水后结晶速度应该与冷水直接冷冻一样。因此,考虑到把温水冷却成冷水时耗费的时间,应该得出结论,即无论怎样冷水都应比温水冷冻得快。看来,这个“温差理论”也不值得推敲。
那么,温水到底缘何比冷水冷冻得快呢?温水在冷冻过程中肯定还有一个至今未被人们认知的机理。也许不久的将来,科学家会解开藏在我们身边的这个谜团。
结冰的核心是要在结冰的过程中有冰晶核心的存在。也许是因为冰激凌的成份中不同组份的凝固温度点不同,因此在稍高温度时由于不均质容易产生一个核心用来作为晶核。比较低温的时候会因为相对均质而不容易产生晶核,就象纯净水一样。
按热力学,赫尔姆茨自由能关系式:F=E-ST,F取最小值或平衡态是系统的内能E与熵S竞争的结果,这里面决定因素就是绝对温度T。
另外:如果是过冷水, 是要长时间来打破原状态(水)的。
现在,在许多解释中最为普遍的理论为:温差理论,即冬天温水比冷水冻得快,是因为温水与周围环境之间的温差大于冷水与周围环境之间的温差,温差大温水中水分子的能量会很快散发到周围环境中。
因为温差比较大, 所以能量流动的速度比较块, 而当温水变成冷水后, 能量流动的速度仍然有惯性, 慢不下来, 因此温水变成的冷水, 失去能量的速度比刚开始就是冷水的那个冷水速度要快, 所以温水结冰比冷水快.
热能的流动是什么造成的? 好像是分子运动吧, 任何运动都有惯性. 所以 ......
过去杂志上东抄西抄乱抄的,结果大家都以为是真的,自己做个实验就知道了
我觉得这还是因为温水散热比冷水快的缘故.
如果在冬季洗温水澡要比冷水澡凉快?即:冬天应该用冷水洗澡才对?
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
(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.
当然不凉快了
时间项可以在某些情况下代入。即使未必证明快慢,至少说明趋势是完全取决于温度的。