If ice sank, Earth's lakes would freeze solid from the bottom up.
Almost every substance is densest as a solid. Water is a rebel: it is densest at about 4°C, and ice is lighter still. So ice floats, forms an insulating lid on lakes and seas, and the water beneath stays liquid enough for life to survive the winter.
For most substances, cooling makes molecules pack more tightly, so the solid form is denser than the liquid and sinks. Water breaks this rule. As liquid water cools, it becomes denser until about 4°C. Below that, it starts to expand again, and when it freezes it expands by about nine per cent.
The reason is the hydrogen bond. Each water molecule is slightly positive at its hydrogen atoms and slightly negative at its oxygen, so neighbouring molecules attract. In ice, these bonds lock the molecules into an open hexagonal lattice with lots of empty space, which is why snowflakes have six-fold symmetry and why ice is less dense than the liquid it came from.
The consequences are planetary. In winter, a lake's surface water cools, becomes denser and sinks, until the whole lake is near 4°C. Then the surface cools further, stays on top, and freezes into a floating lid that insulates the water below. Fish overwinter in liquid water at the bottom. If ice sank, lakes and shallow seas could freeze from the bed upwards and might never fully thaw.
The same expansion splits rocks when water freezes in their cracks, a process called frost weathering, which slowly breaks mountains into soil. It is also why a forgotten bottle bursts in the freezer. Water's oddness is so fundamental to life that it is one of the first things scientists look for on other worlds.
Source: Wikipedia — Properties of water · Text summarised from Wikipedia (CC BY-SA 4.0)
