Water is unusual among substances in that it expands rather than contracts when it freezes. This occurs because of the unique properties of hydrogen bonding in water molecules. In liquid water, molecules move freely and pack relatively close together despite forming temporary hydrogen bonds. As temperature drops toward freezing, molecular motion slows and the hydrogen bonds become more organized and stable.

At the freezing point, water molecules lock into a rigid hexagonal lattice structure. In this crystalline arrangement, each water molecule bonds to four neighbors in a fixed geometric pattern. Crucially, this lattice requires significantly more space than the random, crowded arrangement of liquid water. The molecules are forced to maintain specific distances from one another to stabilize the hydrogen bonds, resulting in them being further apart than in the liquid phase. This increased spacing causes water to expand by approximately nine percent when it freezes.

Because the same mass of water occupies a larger volume in its solid form, ice becomes less dense than liquid water and floats. This property is biologically critical: it means bodies of water freeze from the top down rather than the bottom up, with the ice layer insulating the liquid water beneath and allowing aquatic life to survive winter. If water contracted upon freezing as most substances do, ice would sink and lakes and oceans would eventually freeze solid from the bottom, making survival impossible for many organisms. Water also reaches maximum density at approximately 4°C before beginning to expand as it cools further toward freezing, which further contributes to this top-down freezing pattern.