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Seawater Chemistry

Seawater contains ions that affect its properties, such as density.

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Seawater Chemistry

What is salt?

Besides making food taste better, salt is important for the human diet. Before refrigeration, salt was essential for curing and preserving food. Even in antiquity people built access roads they called "salt roads" so that they could obtain this essential mineral. What is salt? It's what you get when you evaporate seawater!

Composition of Ocean Water

Remember that H2O is a polar molecule, so it can dissolve many substances (Figure below). Salts, sugars, acids, bases, and organic molecules can all dissolve in water.

Graph of elements in seawater

Ocean water is composed of many substances, many of them salts such as sodium, magnesium, and calcium chloride.


Where does the salt in seawater come from? As water moves through rock and soil on land it picks up ions. This is the flip side of weathering. Salts comprise about 3.5% of the mass of ocean water, but the salt content, or salinity, is different in different locations.

What would the salinity be like in an estuary? Where seawater mixes with fresh water, salinity is lower than average.

What would the salinity be like where there is lots of evaporation? Where there is lots of evaporation but little circulation of water, salinity can be much higher. The Dead Sea has 30% salinity — nearly nine times the average salinity of ocean water (Figure below). Why do you think this water body is called the Dead Sea?

Man floating on the Dead Sea

Because of the increased salinity, the water in the Dead Sea is very dense, it has such high salinity that people can easily float in it!


With so many dissolved substances mixed in seawater, what is the density (mass per volume) of seawater relative to fresh water?

Water density increases as:

  • salinity increases
  • temperature decreases
  • pressure increases

Differences in water density are responsible for deep ocean currents, as will be discussed in the "Deep Ocean Currents" concept.


  • Water moving through rock and soil picks up ions that end up as salts in large water bodies.
  • Ocean water contains salts, sugars, acids, bases, and organic molecules.
  • Water density increases as salinity and pressure increase, or as temperature decreases.


1. Streams aren't salty, so why is the ocean salty?

2. In a region of the ocean where evaporation is high, what happens to the density of the water?

3. What would need to happen for the all of the oceans to become more saline?

Going Further 

A very cool visualization tool showing monthly and yearly shifts in global ocean salinity, temperature, nutrients, and other characteristics is available here: http://earthguide.ucsd.edu/earthguide/diagrams/levitus/index.html. "Mercator" refers to the map projection type used there (the old-style flattened map).

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