The density anomaly and its biological meaning

Most substances contract continuously on cooling. Water contracts until 4 °C and then expands on further cooling because the open hydrogen-bonded lattice of ice holds the molecules farther apart than the liquid does.

• Consequence: at 0 °C ice floats and forms an insulating lid on lakes and seas, so the water below stays near 4 °C and aquatic life survives the winter.
• In deep lakes in cold regions, water turns over twice a year because of this density maximum — spring and autumn overturn — which brings oxygen down and nutrients up.
• Without the anomaly, ice would sink, lakes would freeze from the bottom up and freshwater ecosystems would not exist in cold climates.
• Same principle applied: water pipes burst, and rocks crack by frost shattering — a case of mechanical weathering.