Why does ice expand when it freezes?

Ice expands when it freezes due to the unique structure of water molecules and their arrangement in a solid state.

Water is made up of two hydrogen atoms and one oxygen atom, forming a V-shaped molecule. When water is in its liquid state, these molecules are free to move around and are relatively close together. However, as the temperature drops and water begins to freeze, the molecules slow down and start to arrange themselves in a specific pattern. This pattern is a hexagonal lattice structure, which is less dense than the random arrangement of molecules in liquid water.

The hexagonal lattice structure is due to the hydrogen bonds between the water molecules. Each water molecule can form up to four hydrogen bonds with its neighbours. These bonds are at their most stable when they form a 109.5-degree angle, which results in the hexagonal shape. This structure takes up more space than the more random arrangement of molecules in liquid water, leading to an increase in volume and hence the expansion of water when it freezes.

This phenomenon is quite unusual, as most substances contract when they cool. It's also crucial for life on Earth. If ice were denser than liquid water, it would sink to the bottom of oceans, lakes, and rivers during winter, potentially causing entire bodies of water to freeze solid and making life as we know it impossible. Instead, ice floats on the surface, providing insulation that helps to protect aquatic life.