A common chemical process may explain how hot water can freeze more quickly than cold.
A scientist of the University of Washington at St Louis found that what is behind the so-called “Mpemba effect is that the phenomenon① is all to do with solutes.
The Mpemba effect came to be known after he noticed that the sugared② milk he used to make ice cream froze more quickly if it started out hot.
The solutes are calcium and magnesium③ bicarbonate④， which make most drinking water “hard” and when the water is heated， these elements precipitate⑤ to form the solid scale that “furs” up the inside of a kettle.
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 hampers⑥ 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.
The two effects combined could perfectly explain why water that has been heated freezes more quickly than water that has not.
① phenomenonn.现象
② sugarv. 加糖于；使甜
③ magnesiumn. ［化］镁[U]
④ bicarbonaten. ［化］重碳酸盐
⑤ precipitatev. 使突然发生；加速；促使
⑥ hamperv. 妨碍，阻碍
为什么开水结冰快
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。
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。
他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。
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。
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。
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。
