Most Liquids Contract When Frozen
In almost every known chemical substance, cooling causes thermal kinetic energy to diminish, allowing intermolecular attraction to pull atoms closer together. Consequently, solid states are almost universally denser than their liquid counterparts.
Water is a rare, life-sustaining exception to this rule. Between 4°C and 0°C, liquid water begins to expand rather than contract, reaching its maximum density at exactly 3.98°C.
The Geometry of the Hexagonal Crystal Lattice
Each water molecule (H2O) contains an electronegative oxygen atom and two electropositive hydrogen atoms. In liquid form, these molecules constantly tumble, forming and breaking fleeting hydrogen bonds while remaining tightly packed.
As temperature drops below the freezing threshold (0°C), molecular motion slows sufficiently for permanent hydrogen bonds to lock in place. The bonds enforce a rigid, tetrahedral arrangement that forms a hexagonal ring structure with spacious interior voids.