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 does contract when cooled.

Ice is not water. It’s chemically the same but it has a different inter-molecular structure than water. At least, the kind that we typically see does. As a result, the same number of water molecules will take up more volume in ice form than in liquid form.

There are other forms of ice, but they only exist under extremely high or low pressure, so we don’t typically see them. Some of these are more dense than liquid water.

https://en.wikipedia.org/wiki/Ice#/media/File:Phase_diagram_of_water.svg