Seawater is a mixture of liquid water and inorganic molecules present in a range of concentrations. Suspended solids, dissolved solids, and dissolved gases are all present. The salinity, or concentration of dissolved solids, distinguishes it from freshwater. Normal seawater has a salinity of about 3.47% or 34.7 parts per thousand (ppt), a more common means of expressing salinity.

More than 99% of the salinity of seawater results from just six ions. In order of decreasing concentration these are: chloride, sodium, sulfate, magnesium, calcium, and potassium. Because these ions are present in the same relative amounts in any seawater sample, salinity can be determined by measuring just one ion's concentration -- usually chloride. This is routinely done using a salinometer, an instrument that determines chloride concentration by measuring the electrical conductivity of water.

The source of the dissolved ions in seawater is runoff of fresh water from the land surface. Although fresh...