Seawater Properties Notes
Composition
Seawater is made up of water (H2O) and dissolved salts. Liquid water is an excellent solvent because it is polar, so it will dissolve other polar and ionic compounds. Oxygen and hydrogen share electrons to form water molecules (a covalent or molecular bond), but these electrons are held slightly more closely to the oxygen atom. This gives 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 solute it can hold. For 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, 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 denser 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 there gets relatively saltier.
Salinity
The salinity of seawater is a measure of the amount of dissolved solids present in the water. Fresh water has a salinity of 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 3.50/00) 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 seawaterChloride / (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
Summary
Water can pull apart ionic compounds like salt because it is a ______molecule with a ______end near hydrogen and ______end near oxygen.
Because seawater has a number of ions dissolved in it, it is more ______than fresh water.
Seawater doesn’t freeze or evaporate as quickly as fresh water because of the dissolved ______.
When seawater freezes, the ice produced is pure ______because the ions are squeezed out leaving the unfrozen water even saltier.
Important terms to know - define the following terms and give an example for each.
nonpolar molecules / hydrogen bonds / hydrophilic / capillary action / cohesionpolar molecules / amphipathic / hydrophobic / surface tension / adhesion