你相信吗,开水比冷水更快结冰!这就是所谓的姆潘巴现象。这种现象到底基于何种原理呢?下文将会为您揭开答案。
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.
Solute: 溶质
Magnesium:镁
Bicarbonate:重碳酸盐
Precipitate:沉淀
Hampers:阻碍
第二篇:学生为什么逃课英语作文
increasing numbers of college students are cutting classes, which has caused a great deal of social concern and criticism. some think that the larger part of the blame should go to the students. others, however, think that it is the teachers who should be responsible. but i think both the students and the teachers have fault of their own.
compared with middle school students, college students have relatively more "freedom". if they are not strict with themselves, chances are that they will oversleep in the morning, go to the internet or find other activities to entertain themselves during class hours. in addition, some students have the misconception that work experience is more important than theoretical knowledge when it comes to job hunting. so they busy themselves in part time jobs when they should be attending class.
on the other hand, some teachers still conduct classes in the old fashioned way. their classes are so boring that they make many students fall asleep. besides, some textbooks are so out of date that they think it a waste of time attending classes.
in a word, both the students and the teachers should be wide awake to this serious problem and make great efforts to use the time at college to its best advantage.