Ice floats on water because of its unique molecular structure and the behavior of water molecules as they freeze.
Water is unusual in that it is denser in its liquid state than in its solid state. When water freezes, the molecules form a crystalline structure that is held together by hydrogen bonds. This structure is arranged in such a way that there are more open spaces between the molecules compared to liquid water, which allows ice to occupy more volume.
As a result, the density of ice is lower than that of liquid water. Specifically, the density of ice is about 0.92 grams per cubic centimeter, while the density of liquid water at its maximum (around 4°C or 39°F) is about 1.00 gram per cubic centimeter. Because of this difference in density, ice is buoyant and floats on water, demonstrating the principle that less dense substances will float on denser substances.