Sublimation on this moment of science.

 

　　Pop quiz, everybody put away your books, and keep your eyes on your own radio.

 

　　You fill the ice tray up to the rim and put it in the freezer. The ice that results will be

 

　　a.) at the rim

 

　　b.) over the rim

 

　　c.) under the rim.

 

　　Answer? It depends.

 

　　I know, you hate that. But it does depend — it depends on how long you wait before checking on the ice again. My guess would be that most folks chose answer b: over the rim. That is correct, if you checked on the ice trays after only a couple hours. That’s because water expands when it turns into ice, as you know if you ever put a soda bottle in the freezer and came back to find it exploded all over the fish sticks.

 

　　But if you were to leave the ice tray untouched for a couple of months, you might find the ice to be under the rim, or lower than the water was.

 

　　Huh? Didn’t we just say water expands when it freezes? Initially, yes. But frozen water can do something else as well: it can sublimate. To sublimate is to go from one state to another without passing through the usual intermediary state.

 

　　By adding heat, ice can be melted into liquid water, which can be evaporated. That’s three states: solid, liquid, gas. But given enough time, ice can also turn into a gas without melting first! When your ice cubes seem to have shrunk, it’s because they are sublimating: the frozen water is slowly evaporating off the cubes, without ever actually melting.

 

　　Will this work in any freezer? Next time.

 

　　升华现象

 

　　今天我们的节目要讲的是：升华现象。

 

　　各位，请放下你们手中的书本，专心听收音机，我们来做一个突击测验。

 

　　用水将冰格填得满满的，漫过冰格的边缘，然后再将冰格放到冰箱的制冷室里，最后结成的冰会：

 

　　a.) 刚好齐边

 

　　b.) 边缘以上

 

　　c.) 边缘以下

 

　　答案是？要看情况而定。

 

　　我明白，你们不喜欢这样的答案。但是，实际情况确实如此——答案取决于你要过多长时间去查看。我猜，很多人都会选择b选项：边缘以上。如果你在放入制冷室仅两个小时之后去查看的话，这个答案就是正确的。这是因为水在变成冰的时候会膨胀。就像大家所知道的，如果将一瓶汽水放在制冷室中，回头去看的时候会发现，汽水已经炸开了，整个制冷室中到处都是汽水。

 

　　但是，如果你将冰格放在制冷室中，好几个月都不动它的话，你会发现，冰在边缘以下，或是低于你之前装的水的高度。

 

　　哈？我们刚刚不是才说了水在变成冰的时候会膨胀吗？最开始的时候，会。但是冷冻之后的水也会有其他的变化：升华。升华也就是物质不经中间状态直接从一个状态转变成另一个状态。

 

　　通过加热，冰可以化成液态水，液态水可以被蒸发为水蒸气。这是三个状态：固态，液态，气态。但是，如果时间足够的话，冰可以在不被融化的情况下转变为气态。如果你的冰“缩水”了，那是因为被升华了：在没有被融化的情况下，固态水以非常缓慢的速度在蒸发着。

 

　　这种现象在任何的制冷器中都会出现吗？下次再谈。