The density anomaly of water: why ice floats

In short: Most substances become denser as they cool and are heavier as a solid than as a liquid. Water is an exception: it is densest at about 4 °C, and ice is lighter than liquid water.

In more detail: When water freezes, its molecules arrange themselves via hydrogen bonds into an open, hexagonal lattice. This lattice takes up more space than the disordered molecules in liquid water, so water expands by about nine per cent when it freezes.

In detail

• Ice floats: Because ice is less dense than water, it floats on top. With an iceberg only about a tenth sticks out of the water; the rest lies beneath.
• Maximum density at 4 °C: If water is cooled below 4 °C, it already begins to expand slightly again, even though it is still liquid.
• Lakes freeze from the top: In autumn, cold surface water sinks until the whole lake is at about 4 °C. After that the colder, lighter water stays on top and freezes first.
• Surviving winter: The ice cover insulates, and at the bottom of deep lakes the water usually stays around 4 °C. This lets fish and other creatures survive the winter.
• Burst pipes: When water freezes in a pipe or bottle, the expansion can break the material. Pipes in unheated rooms should therefore be drained or protected in winter.
• Frost weathering: When water seeps into cracks in rock and freezes, it widens them. If this happens often, rock gradually breaks apart, and potholes form in roads.
• Other exceptions: A few other substances such as silicon, gallium and bismuth also expand when they solidify, but among everyday substances water is the most striking exception.
• Importance for life: If ice sank, bodies of water could freeze solid from the bottom up. The anomaly is therefore important for life in lakes and seas.
• Connections: Water in its states, floating and weathering (aggregatzustaende, auftrieb-und-archimedes, gesteine-und-gesteinskreislauf).