Our world is full of small wonders. Ice floats because of the nature of the basic particles from which it is made and these are far, far too small for our eyes to see. If these particles were different, ice might sink and you can imagine what a difference this would make to our world.
Water, of course, is made from atoms of hydrogen and oxygen and the basic recipe calls for two atoms of hydrogen for every atom of oxygen. These atoms are bound together in a certain fashion to make the basic particles of water. Two atoms of oxygen and four atoms of hydrogen form a bond by sharing electrons.
The behavior of water depends upon the nature of these basic particles. At normal temperatures, they tend to cling together and flow along in a sort of follow the leader game. The speeding energy they use is heat and as the temperature drops, there is less available energy and the speed is reduced. At 0 degrees centigrade, the motion is so slow that the slithery liquid becomes a stiff solid.
They now stack together in solid form and become ice, But because of their particular shapes, they cannot fit perfectly together. So ice is a delicate lattice of rigid water particles riddled with tiny spaces. These spaces add bulk to the frozen water, but no extra weight.
Imagine two pails of equal size, one brimful of water and the other exactly filled with solid ice. If the water weighs ten pounds, the ice weighs nine pounds one ounce. The ice is not so dense as the water and any substance will float if it is less dense than water.
The less dense it is, the higher it floats.
Since ice is about nine tenths the density of water, one tenth of its bulk floats above the surface. Nine tenths of a threatening ice berg is below the water level.
Most substances become more dense when they freeze solid. As the temperature drops, the speeding particles lose their energy and pack tightly together, shoulder to shoulder. Ice is an exception. Its frozen particles cannot fit neatly together and the tiny spaces between them add the extra bulk which makes frozen water lighter and less dense than liquid water.
The shape of the water particle is one of the many small wonders that make life possible on our planet. If frozen water sank to the bottom, it would have less chance to melt in the warm sun and in then, perhaps, the seas and all the rivers would fill up with pieces of ice. Our warm, watery world would be a wintery planet of frozen glaciers.