Why does ice expand when it freezes?

Ice expands when it freezes due to the unique structure of water molecules and their hydrogen bonding.

When water is in its liquid state, the water molecules are close together, moving around and colliding with each other. However, as the temperature drops and water starts to freeze, something interesting happens. The water molecules slow down and start to arrange themselves in a specific pattern due to hydrogen bonding.

Water is a polar molecule, meaning it has a positive end and a negative end. This polarity allows water molecules to form hydrogen bonds with each other. A hydrogen bond is a weak bond between two molecules resulting from an electrostatic attraction between a proton in one molecule and an electronegative atom in the other. In the case of water, the hydrogen atoms (positive end) of one water molecule are attracted to the oxygen atoms (negative end) of another water molecule.

As water cools and begins to freeze, these hydrogen bonds start to stabilise and hold the water molecules in a fixed position. The arrangement that the molecules take on is a hexagonal structure, which is less dense than the random arrangement in liquid water. This hexagonal structure has a lot of open space, causing the water to expand and increase in volume.

This is why ice floats on water. The ice is less dense than the water because of the open spaces in its molecular structure. This expansion upon freezing is an unusual property. Most substances contract when they cool because the molecules slow down and have less kinetic energy, allowing them to move closer together. But the unique properties of water and its hydrogen bonding cause it to behave differently, leading to expansion when it freezes.