Why does ice have a lower density than water?

Ice has a lower density than water because its molecules are arranged in a hexagonal structure, creating more space between them.

When water freezes and turns into ice, the molecules slow down and arrange themselves in a specific pattern. This pattern is a hexagonal lattice structure, which is less compact than the random, more closely packed arrangement of molecules in liquid water. This hexagonal structure creates more space between the molecules, which results in a lower density.

Density is defined as mass per unit volume. In the case of water and ice, they have the same mass (as the number of molecules doesn't change during freezing), but the volume of ice is greater due to the increased space between the molecules. This means that the density of ice (mass/volume) is less than that of water.

This property of ice is quite unusual, as most substances become denser when they change from a liquid to a solid state. This is because the process of solidification usually allows molecules to pack more closely together. However, the unique properties of water molecules lead to the formation of a hexagonal structure when frozen, which is less dense.

This lower density of ice compared to water has important implications. It's the reason why ice floats on water. If ice were denser than water, it would sink, which could have significant effects on aquatic life and the Earth's climate. For example, if ice sank, it could lead to the freezing of entire bodies of water from the bottom up, which would be detrimental to aquatic ecosystems.