探秘--为什么开水结冰快

A common chemical process may explain how hot water can freeze more quickly than cold, a report on the web site of New Scientist said on Thursday.

A scientist of the University of Washington at St Louis found that what is behind the so-called "Mpemba effect is that the phenomenon is all to do with solutes," the report said.

The Mpemba effect came to be known after a Tanzanian school student named Erasto Mpemba noticed that the sugared milk he used to make ice cream froze more quickly if it started out hot.

Jonathan Katz, of the University of Washington, who worked out the details of the Mpemba effect, said the solutes are calcium and magnesium bicarbonate, which make most drinking water "hard" and when the water is heated, these elements precipitate to form the solid scale that "furs" up the inside of a kettle.

Katz said water that has never been heated still contains these solutes and as it freezes, ice crystals form, and the concentration of solutes in the remaining water becomes ever higher - up to 50 times as high as normal, thus lowering the freezing point of the water.

According to Katz, there is a second, related effect that hampers the freezing of water that has never been heated. The lowering of the freezing point reduces the temperature difference between the liquid and its freezing surroundings. "Since the rate at which heat is lost from the water depends on this temperature difference, water that has not been heated has greater difficulty losing heat," Katz was quoted as saying.

Katz said the two effects combined could perfectly explain why water that has been heated freezes more quickly than water that has not.

Katz is waiting for someone to do experiments to test his theory, New Scientist said. 

中文翻译：

一个普通的化学操作将揭示为什么热水能比冷水更为迅速地结冰，星期四，一项来自新科学家的网站的报告称。

该报道称，一位来自位于圣路易斯的华盛顿大学的科学家，发现了隐藏在所谓的姆潘巴现象的实质，姆潘巴现象就是该现象的产生和溶质有关。

来自华盛顿大学的Jonathan Katz，发现了姆潘巴现象的细节。他说，溶质是钙和镁的重碳酸盐，这些物质使很多饮用水变得很“硬”，当水被加热后，这些物质沉淀生成了固体物质，并在水壶内壁形成水垢。

Katz表示，没有被加热过的水仍然含有这些溶质，当它被冷冻，冰晶形成的时候，这些仍保存在水中的溶质浓度变得更高了，大概高出正常的50倍，这样就降低了水的冰点。

据Katz称，还有另外一个原因阻止未被加热的水结冰。低冰点缩小了固体和其结冰环境下的气温差异。“因为热量从水中流失的速度取决于温差。没有被加热过的水更难流失热量。”

Katz说这两个原因结合在一起就可以很好地揭示为什么热水比冷水更容易结冰了。

Katz正等待有人能通过实验来证实他的理论。新科学家称。