Physical properties of seawater

T he world ocean (BoxA) covers more than 70% of planet Earth's surface and contains 97% of its water reserves (which amount to around 1,400 × 10 ⁶ km ³ ). A dozen major ionic species are present in seawater. Their total mass may vary from one seawater to another, but their relative proportions remain constant. Seawater can thus be unambiguously characterized by its salinity. The average salinity of the world's oceans is around 35 and their temperature 4°C.

Given its relatively stable composition and enormous volume, seawater is an original electrolytic solution and deserves more attention from physical chemists. This is not yet the case, and the various constants handbooks provide little or no information on the physical properties of seawater or on thermodynamic equilibria in the seawater medium.

Seawater is a physical medium perfectly defined by three state variables: salinity S, temperature t and pressure p. All its physical properties are therefore, in principle, derivable from S, t and p. In classical physical chemistry, we generally refer to a temperature of 25 C, and a normal pressure of 101,325 Pa (or 100 kPa if we speak of standard pressure). In oceanography, the standard ocean (Box A) has a temperature of 0°C and a salinity of 35,000. But its pressure is not standardized, because a body of water can be just as normally located at a depth of 10,000 m as at the surface.

The effect of high pressure on the physical properties of solids or liquids is difficult to study experimentally and is often poorly documented. This is not the case in the field of oceanography, where a particular effort has been made in this direction. Most oceanographic algorithms can accurately calculate the hydrological properties of seawater as a function of S, t and p, for salinities ranging from 0 to 42, temperatures from –2 to 40 C and pressures from 1 to 10,000 dbar. However, these algorithms cannot be used for inland seas, such as the Caspian Sea or salt lakes. These bodies of water have a different salt content to seawater.

Many people are familiar with the names and definitions of some of the properties described in this article. These include density and the speed of sound. Other properties will seem more mysterious to non-specialists, such as thermosteric anomaly or adiabatic decay rate. These properties are worth mentioning, as they form part of oceanographic data processing routines.