Seawater Properties

Composition
Seawater is made up of water (H2O) and dissolved salts. Liquid water is an excellent solvent because it is polar. Oxygen and hydrogen share electrons to form water molecules (a covalent bond), but these electrons are held slightly more closely to the oxygen atom. This give the oxygen atom a partial negative charge and the hydrogen atoms partial positive charges. The partial charges on water molecules allow them to pull apart and dissolve ionic compounds (like salt) very easily.

As good a solvent as seawater is, there is a limit to how much of a given salt it can hold. For regular salts like halite (NaCl) and sylvite (KCl), that limit is very high, but for some materials like iron (Fe) and silver (Ag) that limit is low. When water has dissolved as much of a given compound as it can hold, the water is saturated with respect to that compound. If more of the compound is added to the water, it will not dissolve but will just sink to the bottom. If some of the water is removed by freezing or evaporation, the dissolved ions will form solid crystals and sink to the bottom. This is called precipitation.

Density
Density is a measure of how mass is distributed. A pound of feathers and a pound of lead have the same weight, but the pound of feathers takes up much more space, because feathers are less dense.

Fresh water has a density of one gram per cubic centimeter (1 g/cm3). Seawater is more dense than fresh water, because the ions dissolved in seawater are made up of atoms that are denser than hydrogen and oxygen, the components of water.

Temperature
Fresh water freezes at 0°C and boils at 100°C. The dissolved ions in seawater make it harder to freeze, because the ions get in the way when the water molecules arrange themselves into ice crystals. This means that seawater has a lower (colder) freezing point than fresh water. Likewise, the dissolved ions also make seawater harder to evaporate.

For both freezing and evaporation of seawater, the ions will stay in the liquid water. Ice and water vapor aren't good solvents at all; they can't hold the ions. So, when water freezes at the poles, the ice is pure while the sea water it forms from gets relatively saltier.

Salinity
The salinity of seawater is a measure of the amount of dissolved stuff present in the water. Fresh water has a salinity near zero, while the oceans range in salinity from about 3.3% to 3.7% by weight. That is, of every 100 grams of ocean water, about 3.5 of them are actually made up of dissolved ions, while 96.5 of them are water. Oceanographers and marine geologists normally refer to salinity in units of parts per thousand. 3.5% is the same as 3.5 parts per hundred, which is the same as 35 parts per thousand. So, a salinity of 35 parts per thousand (written 35 o/oo) is pretty typical for most of the ocean.

Dissolved Ions in Seawater

Dissolved Ion

Chemical Formula and Charge

% by weight of dissolved ions

% by weight of seawater

Chloride

(Cl-)

55.04

1.898

Sodium

(Na+)

30.61

1.0556

Sulfate

(SO42-)

7.68

0.2649

Magnesium

(Mg+)

3.69

0.1272

Calcium

(Ca2+)

1.16

0.04

Potassium

(K+)

1.1

0.038

Bicarbonate

(HCO3-)

0.41

0.014

Bromide

(Br-)

0.19

0.0065

Boric Acid

(H3BO3)

0.07

0.0026

Strontium

(Sr2+)

0.04

0.0013

Fluoride

(F-)

0.002

0.0001

Total

99.992

3.4482
Seawater ions plot

Principle of Constant Proportions
One of the neatest things about seawater and its dissolved ions is that they always come in the same ratios. For example, if you have seawater with salinity 32 parts per thousand, it will have the same ratio of chlorine to sodium (Cl/Na = 55.0/30.6 = 1.8) as seawater with salinity 38. The salinity 38 water will have more of each of Cl and Na and the other ions, but they will still have the same relative proportions. This principle holds true in space; all over the world, the ions exist in the same ratios. It also holds true in time. Ancient seawater appears to have had the same proportions of different dissolved ions in it that modern seawater does.