點解冰總是會浮在水面??

在冬天，只要溫度計的水銀柱下降到0oC以下水就會結冰。

大多數物體都是熱脹冷縮的。水在4 oC以上的時候，也是熱脹冷縮，但是當它在4 oC以下的時候，溫度愈低，它的體積反而膨脹，直到結成冰為止。由於膨脹，冰就比同體積的水要輕一些。因此，冰總是浮在水面上，而且總是水面上先結冰。

應該說，冰的這種怪脾氣，對人類是很有好處的。要是冰和別的物體一樣，也是熱脹冷縮的話，那麼，天一冷，水面上結成的冰會不斷向下沉，到了最後，江河、湖汨裹的水，會連底凍起來。

寒冷的冬天，河面上往往結著很厚的冰，甚至人還可以在上面走路或進行滑冰運動。但在冰下面的水裡，魚和蝦能照樣游動。為甚麼魚蝦不會被凍死呢，就是由於4 oC的水保護了它們。

因為密度唔同嘛！

The main reason is ice has a lower density than water and thus experienced an upthrust which is greater than its weight. Ice thus floats.

Let dw be the density of water, di be the density of ice, and m be the mass of an ice cube.

The volume of water displaced by the ice cube if it is fully submerged in water
= volume of ice
= mass/density = m/di

By Archimedes Principle, upthrust = weight of water displaced
thus, upthrust = (m/di)dw.g
where g is the acceleration due to gravity

The net force acting on the ice cube
= (m/di)dw.g - mg
= mg(dw/di-1)

since dw is greater than di, thus the fraction dw/di is greater than unity, the net force is positive. That is, the upthrust acting on the ice cube is greater than its own weight, the ice cube will floar.

因為冰的密度小於水的密度，當一定體積的水結冰後，它的體積會曾大１／１０所以密度自然就小了，也就浮起來了．

因為水既質比冰低~