Eli5: Why does water expand while freezing, and what would happen if it was frozen in pressurized enviroment/container that cannot expand?

Question

Eli5: Why does water expand while freezing, and what would happen if it was frozen in pressurized enviroment/container that cannot expand?

In: Physics

Answer 1

> what would happen if it was frozen in pressurized enviroment/container that cannot expand?

There’s no such thing as a container that doesn’t expand as the internal pressure increases. The only way to achieve zero volume increase would be to actively increase the external pressure to compensate.

As the ice froze, the pressure would increase and this would compress the ice, keeping it smaller than it would be at normal pressures. The common water ice we see is hexagonal but there are many other water ice “phases”. Some of those that form at high pressures are denser than liquid water.