Why Does Water Expand When It Freezes?

Most substances contract when they freeze because molecules pack more tightly in the solid state. Water is anomalous because of hydrogen bonding. In liquid water, hydrogen bonds constantly form, break, and reform, allowing molecules to move relatively close together. When water cools below 4 degrees Celsius (its maximum density point) and crystallises at 0 degrees Celsius, hydrogen bonds lock each molecule into a fixed position in a hexagonal lattice. This lattice has large open spaces between molecules, making ice about 9% less dense than liquid water (917 kg/m3 vs 1,000 kg/m3). This expansion is why ice floats, pipes burst in winter, and frost can crack rocks. The fact that ice floats is ecologically crucial: it insulates lakes and oceans from the top down, allowing aquatic life to survive underneath. If ice sank, water bodies would freeze from the bottom up, potentially killing most aquatic organisms.