Question

why water expand upon freezing ?

Answer 1

water molecules are polarised giving water some awesome properties such as a v high capacity to store energy as electric potential energy between its liquid charged particles.

but as water cools (and contracts), a point is reached where individual molecules of water will align in a fixed [hexagonal] structure, as opposed to water's normal fluid structure which is based on hydrogen bonding between polarised H2O molecules.

a body of water will lose energy in doing this but it will also increase in volume, meaning it is less dense.

in terms of the "why", my best answer is entropy, simply because energy is dissipated in this process. but that, even if it is correct, begs the question as to why entropy always increases as a law of nature.