Why does water expand when it freezes?
Ordinary ice formed at normal pressure occupies more volume than the same mass of liquid water. Hydrogen bonding produces an open crystalline structure. Its lower density allows ice to float on liquid water.
Changing molecular arrangement
Hydrogen bonds in liquid water continually rearrange, allowing closer average packing. Freezing creates a more open arrangement. Liquid water reaches its greatest density near 4 °C; further cooling below that temperature lowers its density. This behavior is not universal among substances, and high-pressure forms of ice can have different density relationships.
Consequences in nature and structures
Surface freezing helps lakes retain liquid water below, while the ice cover reduces heat transfer. Freezing in confined pipes can cause damage through expansion and pressure buildup. Repeated freezing and thawing in rock cracks contributes to physical weathering. Dissolved salts change freezing behavior, so seawater does not freeze at the same temperature as pure water. Purity, pressure, and temperature must all be considered.