The mass per unit volume of a substance is called the volume of a substance increases its density decreases.though ice is a solid,it has large number of empty spaces between its particles.these spaces are larger as compared to the spaces present between the particles of water.hence  the density of ice is less than that of water . a substance with lower density than water can float on water.therefore ice floats on water.
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Ice which is a solid has vacant spaces between water molecules thus making ice lighter than water. Thus ice floats on water.
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