Eli5: Materials expand when heated and contracts when cooled. So why does water expand when it freezes instead of contracting?

Question

Eli5: Materials expand when heated and contracts when cooled. So why does water expand when it freezes instead of contracting?

In: 24

Answer 1

Water is a little bit ionic. The atoms are slightly negatively charged on the oxygen side and slightly positively charged on the hydrogen side. You can show this with [a science experiment](https://www.youtube.com/watch?v=HpIJ3cZURpQ) involving static electricity.

When freezing, something interesting happens. This effect causes water, vibrating much less as it cools, to align its molecules with each other. This actually causes ice to expand a bit since the aligned molecules now have limitations on how they want to fit together.