What characteristic makes ice less dense than liquid water?

Ice is less dense than liquid water primarily due to the structure formed by hydrogen bonding during the freezing process. When water freezes, the molecules arrange themselves into a crystalline lattice structure, which stabilizes distances between the molecules in such a way that there is more empty space in the ice than in the liquid form of water. This molecular arrangement makes ice occupy a larger volume compared to the same mass of liquid water, resulting in lower density.

In contrast, in its liquid state, water molecules are in constant motion and can slide past one another, allowing them to be closer together, which increases density. This unique property of water, driven by the hydrogen bonds, is crucial as it enables ice to float on liquid water, which has significant ecological implications for aquatic life during winter months. Temperature effects, ionization, and evaporation rates do not directly account for the lower density of ice compared to liquid water; rather, it is the hydrogen bonding that plays a central role in this characteristic.