Why is the density of ice less than that of water?
Ice is less dense than water because as water cools and becomes a solid (freezes), hydrogen bonds form between the water molecules. In the liquid phase of water, the molecules “snuggle up” to each other in the fluid. But as water goes solid, the hydrogen bonds dictate that the molecules will have to “stop snuggling” and move apart a bit as those hydrogen bonds set up spacing in the now-solid molecules. Ice has become less dense than the water that it formed from because the hydrogen bonds, which begin forming at just above 0 °C, force the molecules apart a bit to form the solid (ice) matrix.
