The mass per unit volume of a substance is called density ( density = mass/volume ). as the volume of a substance increase ,its density decrease .though ice is a solid , it has large number of empty space between its particles . these spaces are large as compared to the space present between the particles of water .Thus , the volume of ice is greater than that of water .Hence ,the density of ice is less than that of water .  A substance with lower density with lower density than water can float on water . Therefore ice float on water .