Any thing which float or sink never depend upon that the given thing is light or massive,it depends upon relative floats on the surface of water because density of ice is less than the density of water and another reason is that water has maximum density at 4 degree Celsius but water become ice at 4 degree Celsius means density of water decrease when it acquire ice form and become less denser than water. So ice floats on the surface of water.
This happens due to anomalous expansion of water. When water is cooled below 4 degree celsius and when it becomes solid(ice), its density starts to decrease instead of increasing and hence, it floats on water whose density is more. The behavior of any substance that is contrary to the general trend is called anomalous behavior and this behavior of water is called anomalous expansion.

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