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

When freezing the molecules arrange themselves in a fixed pattern, That pattern is more spread out than liquid water, thus expanding the volume. When cooling water below zero in a container part of the water would freeze increasing the pressure in the container and preventing more water to freeze. The cooler the water the more of it freezes. You can look at the phase diagram to see how much pressure is needed to keep water liquid under certain temperatures.