Anomalous Water Expansion

Water has ANOMALOUS thermal expansion: density INCREASES as it COOLS from 4°C down to 0°C (volume increases).

Density MAXIMUM at 4°C: ρ = 1000.0 kg/m³.

Below 4°C: water EXPANDS as it cools further (negative γ).

When water freezes at 0°C: volume EXPANDS by ~9% (ice less dense than water).

Why ice FLOATS on water: ice (ρ ≈ 917 kg/m³) < water (ρ = 1000 kg/m³).

Lakes freeze TOP-DOWN: dense 4°C water sinks; cold (less dense) water and ice float on top.

Result: aquatic life survives under ice — bottom water stays at 4°C through winter.

Origin: hydrogen bonding in water creates open hexagonal structure in ice, denser packing in liquid.