Why does water expand when frozen whilst other liquids contract?

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Why does water expand when frozen whilst other liquids contract?

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Because of the crystal structure the water molecules form when water freezes. Due to their shape and properties, the most happy solid shape waters can take is a hexagonal (honeycomb) lattice [that looks like this.](https://crystalsymmetry.files.wordpress.com/2018/04/ice_ih_molecular_arrangement.png) On the top left is one water molecule. On the right is how waters are arranged in ice. (Bottom left just shows *why* they make hexagons, due to the (+)s and (-)s attracting each other.)

When water is a liquid, it’s like having just a [bunch of these](https://media.sciencephoto.com/image/a7000381/800wm) loose in a bucket, not connected to each other.

I’m hoping you can imagine that if they were all loose and free to move around, they’d pack together more densely then if you assembled the same number of waters into the hexagon-shaped framework from my first picture. The hexagons are less dense, there’s more open space.

So when water freezes, it goes from a bunch of loose, non-connected waters to a rigid frame of water hexagons that takes up more space…the water has expanded.

When most things freeze, they go from a random arrangement of molecules/atoms to a crystal, which has a regular arrangement of molecules/atoms in a rigid framework.

In almost all materials, the solid crystal is more compact than the liquid arrangement, hence most materials shrink when they freeze.

Water, due to a fairly unique molecular shape and charge (it’s a compact, polar, and “bent” molecule), has a crystal structure that pushes the molecules a little farther apart than they are on average when they’re a liquid. Hence expansion on freezing.

Tl;dr: Water has a fairly unique molecular structure that results in the solid crystal being slightly less dense than the liquid form.