Q.Describe the anomalous expansion of water between $0\,^\circ\text{C}$ and $4\,^\circ\text{C}$. Explain, using this property, why a pond in a cold climate freezes only at its surface while the water at the bottom remains liquid at about $4\,^\circ\text{C}$, allowing aquatic life to survive through the winter.

As water is cooled towards $0\,^\circ\text{C}$, it contracts (density rises) in the usual way only down to $4\,^\circ\text{C}$; cooling it further, from $4\,^\circ\text{C}$ to $0\,^\circ\text{C}$, makes it expand again instead, so its density actually falls as it approaches the freezing point. Water is therefore densest at exactly $4\,^\circ\text{C}$ -- this is the anomalous expansion of water.

In a pond, as air temperature falls, the surface water cools and (while still above $4\,^\circ\text{C}$) sinks because it is denser, mixing the pond and cooling it fairly uniformly down to $4\,^\circ\text{C}$ throughout. Once the whole pond reaches $4\,^\circ\text{C}$, further cooling of the surface layer makes that layer less dense than the water below it, so it no longer sinks -- it stays at the top and continues cooling until it freezes into a floating ice sheet. The w …

Explain the density-temperature curve (maximum at $4\,^\circ\text{C}$) and trace through why convective mixing in a cooling pon …

• Saying ice is denser than liquid water (it is the opposite -- ice floats because it is less dense). …