Why does ice float?
(Lansing State Journal 4/8/98)
In order to answer this question we first have to understand why anything floats. This question is easiest to understand in the case of water. When you put a bunch of marbles into a bowl of water, you can see the level of the water rise. In fact , the marbles are pushing the water out of the way, that is, displacing the water. The marbles sink because they are more dense than the water. In order for an object to float, it must be able to displace a weight of fluid equal to its own weight. The amount of water that is displaced by a marble weighs less than the marble, so the marble sinks.
As we discussed last week, water expands when it freezes, becoming
ice. So when an ice cube floats in a bowl of water, the ice displaces
an amount of water equal to its weight, so it floats. This property
of ice is very important since the top of a lake can freeze without sinking.
If the ice sunk, then the entire lake would fill up with ice and all the
fish would be frozen.