Seawater Composition


Seawater is a solution of salts.

When a mineral such as NaCl dissolves in water, the ions separate and are surrounded by water molecules. The lone pair electron orbitals on the oxygen of water interact with the electron-poor cations and the anions donate some of their excess electron density to the electron-poor hydrogen atoms of water.

The salinity of seawater is measured by the number of grams of ions in a kilogram of the water. For seawater of salinity 35, the major ions are:

Ion Concentration (mol/L) g/kg seawater
Cl- 56 x 10-2 19.353
Na+ 48 x 10-2 10.784
SO42- 2.9 x 10-2 2.712
Mg2+ 5.4 x 10-2 1.284
Ca2+ 1.1 x 10-2 0.412
K+ 1.0 x 10-2 0.399


The density of seawater depends increases with increasing salt content and decreases with increasing temperature. This is a key influence on ocean currents.

Colligative Properties

Some properties of a salt solution like seawater are due to the number of particles (anions + cations) that are present. These are the colligative properties.

Pure liquids, including water, are in equilibrium with the gas phase molecule. Consider what would happen if we added some liquid water to an empty flask. Water molecules on the surface with sufficient energy would escape to the gas phase. Gas phase water molecules would condense and become part of the liquid water. Evaporation and condensation processes would continue rapidly but the concentration of water vapor above the liquid water would reach a constant value, the equilibrium vapor pressure. This vapor pressure would increase with increasing temperature and decrease with increased external pressure.

When substances that can't evaporate are dissolved in water, the concentration of water in the liquid is reduced and the equilibrium concentration of water in the gas phase is reduced.

Partial pressure of water, the part of the total gas pressure that is due to water vapor, is a measure of the concentration of water gas.

The vapor pressure of any solution, Pi, is equal to the product of the vapor pressure of the pure solvent, P0i, and the mole fraction of the solvent.

In pure water, all of the molecules in the liquid are water molecules so the mole fraction is 1 (100 % H2O, 55 mol/L). In sea water, the concentration of water molecules in the solution is less than that of pure water so the vapor pressure of sea water is also lower. The vapor pressure of pure water at 20 deg C is 0.0231 atm while the vapor pressure of seawater with the composition listed above is 0.0226 atm.

There are several other colligative properties that we see in seawater:
  1. Boiling point elevation
      The higher the concentration of ions (or molecules) in solution, the higher is the boiling point of the solution. For seawater with a salinity of 35, the boiling point is higher by about 0.3 deg C.
  2. Freezing point depression
      Seawater freezes at a temperature about 2 deg C lower than freshwater.

Professor Patricia Shapley, University of Illinois, 2011