Water H₂O • Density anomaly (12 molecules)
State:
Liquid-like
Above 4 °C:
Water molecules are closely packed and move randomly, allowing the liquid to occupy a smaller volume and remain relatively dense.
Below 4 °C (freezing transition):
Hydrogen bonds force water molecules to lock into an open, ordered network.
This open structure occupies a larger volume, causing the density to decrease as liquid water transforms into ice.
Why this property is crucial for life on Earth:
When water freezes, this open molecular structure makes ice less dense than liquid water,
causing ice to float. Floating ice forms a protective, insulating layer on lakes and oceans,
preventing the water below from freezing solid.
This insulation allows fish, plants, and microorganisms to survive through winter,
stabilizes aquatic ecosystems, and helps regulate Earth’s climate.
Without this unique density anomaly of water, many freshwater environments would freeze completely,
making life as we know it impossible.