Water is Weird

Water is weird. The gas is far from ideal. The liquid is odd. The solid is bizarre.

Ice, the familiar solid form, packs water molecules in an open crystal structure. The oxygen atoms are bound into hexagons held together by bridging hydrogen atoms. The whole assembly looks like a microscopic honeycomb. There's lots of empty space within. Add a little heat and the bridging bonds shake and begin to break. The lattice structure collapses. The shards tumble into the voids. The ice has melted. More heat breaks the fragments up further. The smaller bits fill the open spaces better, and liquid water contracts as it warms. It reaches its peak density at 4°C (39°F) and then expands like a normal liquid after that.

Molten ice is denser than its solid form. So ice floats on water. (Other solids precipitate in their liquids.) Ice displaces its own weight in its water host—exactly the volume it will have when it melts. That's why melting ice doesn't overflow your glass, and why melting icebergs don't raise sea level.

Liquid water expands when it freezes. There's a considerable force required for that. It's strong enough to shear rocks, or burst water pipes. When outside pressure becomes too high, liquid water remains liquid. Its freezing point drops. The continent of Antarctica, and the moons Europa and Enceladus are blanketed with miles of ice. There, rivers and lakes of liquid water flow beneath tons per square inch of ice overhead. Pressure can melt ice—that's how ice skates work. Put your entire weight on the area of the blade and you exert enough pressure to melt the ice beneath. You glide on that film of wet ice.

Common ice is only one of many forms of crystalline water. Water molecules are like a triangular Lego blocks. They lend themselves to assembly into a variety of shapes. Today, seventeen stable or metastable crystalline ice structures have been identified—most have been created in the lab. Two low-temperature solid forms exist at normal pressure. A dozen more ice crystal forms occur under higher pressures. The full map of their preferred temperatures and pressures resembles a patchwork quilt. Water may exist naturally in some of those forms on giant planets.

Water is everywhere in the universe. Most of it is in none of these forms. Transition into and between these ordered forms takes time and energy. Most extraterrestrial objects are too cold for that to happen. Water molecules or droplets are flash frozen on capture in space—an amorphous blob results. This disordered mass is a supercooled liquid. It's cold and it's hard; it's just another part of the strangeness of water.