Fig 4Why ice floatssame number of moleculessame molecules, 9% less room
hydrogen bondIn ice every water molecule makes four hydrogen bonds at fixed angles, and that locks the molecules into open hexagonal rings with a lot of empty space inside. When ice melts, some of those bonds break, the rings collapse, and the molecules pack about 9% closer. So liquid water is denser than ice, and ice floats. Almost every other substance is densest as a solid.