Advanced #20

Pressure reduces the temperature of maximum density

With increasing pressure, the temperature of maximum density shifts to lower values.

Scientific Explanation

At atmospheric pressure, water reaches its maximum density at 3.98 degrees Celsius — one of its most famous anomalies. Less well known is that this temperature of maximum density (TMD) decreases with increasing pressure. At about 200 megapascals, the TMD has shifted below the freezing point and eventually vanishes entirely.

This behavior can be understood through the interplay of two structural populations in liquid water. At low temperatures and pressures, the open, tetrahedral hydrogen bond networks dominate — these have a lower density. As temperature rises, some of these bonds break and denser, less ordered structures take over. At 3.98 degrees Celsius there is a balance: the increasing thermal expansion is exactly compensated by the collapse of open structures.

Under pressure, the open tetrahedral structure is compressed even at lower temperatures. This means the point at which the two effects balance shifts to lower temperatures. At sufficiently high pressure, there is no density maximum at all because the tetrahedral structure has already been completely destroyed.

Temperature of Maximum Density vs Pressure Line chart showing how the temperature of maximum density of water decreases from about 4 degrees C at atmospheric pressure to lower values as pressure increases, eventually disappearing at around 200 MPa. T of max density (°C) 4 2 0 -2 0 50 100 150 200 Pressure (MPa) 3.98 °C at 1 atm vanishes Temperature of Maximum Density vs Pressure
The temperature of maximum density of water decreases with pressure and vanishes at about 200 MPa.

Everyday Relevance

For deep lakes and oceans, this pressure dependence has practical significance. In a lake, surface water warming in spring reaches peak density at 3.98 degrees Celsius and sinks, driving spring turnover. In very deep bodies of water under higher pressure, this density maximum shifts to lower temperatures, so stratification and circulation patterns differ from those in shallow lakes. For ocean circulation and the survival of deep-sea organisms, these subtle pressure effects are quite relevant.