Contrary to popular belief, the chemical makeup of everyday seawater is much more complex than simple H2O (Water) and NaCl (Table salt). Dozens of other elements are found in varying amounts, dissolved within the sea water.
These four elements are the most abundant elements in seawater, followed by magnesium, sulfur, potassium, calcium and bromine in descending order of abundance. Many more seemingly rare elements are also found in sea water, including gold and uranium. Almost all the elements in the periodic table are contained within sea water to some (often minuscule) degree. Gold for instance, is present in roughly 0.000011 parts per million. This mean there is roughly 1 part of gold in every 100,000 million parts of water.
However, seawater’s chemical composition varies from different sources around the world. Most notably the salinity (amount of salts dissolved) varies from 3.2% up to 4%. This occurs partially due to the amount of fresh water from rivers that is flowing into the seawater, but other aspects such as freezing and thawing can affect salinity. The salinity of seawater is essential to the marine life, due to the process of osmosis in cells, which is affected by the concentration of the water. It is for this reason that fresh water fish often cannot survive in sea water, but also can be true for different areas of sea water with different salinity. This is especially true for seaweed, as they cannot control effectively the osmosis in their cells.
Density of the seawater is also affected by salinity. . The saltier the water is, the higher its density. Pure fresh water’s density is 1.00 kg/l. The density of 3.5% salinity seawater at 15ºC is roughly 1.0255. As substances expand at higher temperatures, the density of water decreases at higher temperatures.
The main gases that comprise air are also dissolved into seawater, although the proportions are different in seawater compared to that of air. Nitrogen comprises approximately 47.5% of the dissolved gases, whereas 36% is oxygen, 15.1% is carbon dioxide and 1.4% is argon. Carbon dioxide therefore, is 500 times more abundant in seawater than in air. The amount of gas dissolved in seawater decreases as salinity increases because the salt ions immobilize water molecules. As temperature increases, the heightened mobility of the gas molecules causes them to escape the water more easily.
Acidicty of seawater is also different from that of fresh water and varies from sources around the seas. The average pH of seawater is roughly 8.14, but local factors can change this greatly. Seawater in deep estuaries may be as acidic as 7.3 pH, whereas in tide pools the seawater can be a strong alkali 9.5.
