Water expands when it freezes due to the way hydrogen bonds reorganize water molecules into a crystalline structure. In liquid water, molecules move freely and pack relatively closely together, with hydrogen bonds constantly breaking and reforming [1][2]. As temperature drops toward 0°C, molecular motion slows and hydrogen bonds become stable and permanent [2]. The molecules then arrange themselves into a rigid hexagonal lattice pattern where each water molecule bonds to four neighbors in a fixed, open framework [1][2]. This geometric arrangement forces molecules farther apart than they were in the liquid state, creating more empty space between them [1][2].

The expansion is substantial—water increases in volume by approximately 9% when it freezes, making ice roughly 9% less dense than liquid water [1][2]. This unusual behavior contrasts sharply with most other substances, which become denser and more compact when they freeze [1][2]. The density difference explains why ice floats on water rather than sinking [1][2].

This property has profound ecological importance. Because ice floats, it forms an insulating layer on the surface of lakes and oceans, protecting aquatic life beneath from freezing temperatures [1][2]. Additionally, liquid water reaches maximum density at approximately 4°C and then becomes slightly less dense as it cools further to the freezing point [2]. This causes lakes and ponds to freeze from the top down rather than from the bottom up [2]. Without water's expansion upon freezing, ice would sink, and bodies of water would gradually fill with ice from the bottom up, making aquatic life impossible [1][2]. The expansion also has practical consequences: it causes frozen water pipes to burst and bottles to break in freezers [1].