Ice floats on water because a piece of ice is lighter than the same amount of water. The ice is not very much lighter than the water, so it floats low in the water.
When water freezes to ice, the water molecules line up in rows. But as they do so, the molecules move apart slightly. The ice increases in size as it forms and this gives ice a lower density than water, making it float in water.
The force between the molecules is very strong, and nothing can resist the expansion that occurs as the ice forms. This is why water pipes sometimes bursts when the weather gets warmer, the ice melts and water pours from the cracked pipe. It is unusual for a liquid to expand on freezing, but it is a good thing that water does so. The layer of ice that may form over lakes and the sea in winter prevents the water beneath from freezing.
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