Anomalous Expansion of Water and Its Effects

Anomalous Expansion of Water and Its Effects

The general rule that a liquid's volume increases steadily as it is heated, and correspondingly decreases

steadily as it is cooled, is one that ordinary water spectacularly breaks over a narrow but critically

important temperature range.

The anomaly

If water at, say, room temperature is cooled steadily, its volume does indeed shrink (density rises) in

the expected way, all the way down to $4\,^\circ\text{C}$. But on cooling further, from

$4\,^\circ\text{C}$ down to the freezing point at $0\,^\circ\text{C}$, water does something unusual: its

volume, instead of continuing to shrink, actually starts to increase again -- equivalently, its density

decreases again as it approaches $0\,^\circ\text{C}$, rather than reaching its highest density right at

the freezing point as most substances do. Water's density therefore reaches an absolute maximum exactly at $4\,^\circ\text{C}$ (very close to $1000\ \text{kg m}^{-3}$), and falls off on both sides

of that temperature -- both as water is warmed above $4\,^\circ\text{C}$ (the ordinary behaviour) and as

it is cooled below $4\,^\circ\text{C}$ (the anomalous behaviour). This departure from the normal pattern

is called the anomalous expansion of water, and it arises from the way hydrogen-bonded water

molecules begin arranging themselves into a more open, cage-like structure as ice formation is

approached, which occupies more volume than the more randomly packed arrangement typical of liquid water

at slightly higher temperatures.

Why lakes freeze from the top down

This one property of water has an outsized effect on how bodies of water in cold climates behave through

winter, and hence on whether aquatic life can survive at all. As air temperature falls, the surface layer

of a lake or pond cools first. Provided that surface layer is still above $4\,^\circ\text{C}$, cooling it

makes it denser than the water below, so it sinks, and warmer water rises to take its place at the

surface to be cooled in turn -- a convective circulation that keeps mixing and cooling the whole body of

water fairly uniformly. But once the entire body of water has been cooled down to $4\,^\circ\text{C}$,

this circulation stops, because any further cooling of the surface layer below $4\,^\circ\text{C}$ now

makes that layer less dense than the $4\,^\circ\text{C}$ water beneath it (the anomalous region), so it

no longer sinks -- it stays at the top. The surface layer can then continue cooling, undisturbed by

mixing, all the way down to $0\,^\circ\text{C}$ and freeze into a solid ice sheet floating on top of the

still-liquid water below.

The consequence is that the bulk of the water beneath the ice sheet remains liquid, and stays close to

its temperature of maximum density, $4\,^\circ\text{C}$, insulated from the much colder air above by the …