13.2 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.

Salinity

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?

In some areas, dense saltwater and less dense freshwater mix, and they form an immiscible layer, just like oil and water. One such place is a "cenote", or underground cave, very common in certain parts of Central America.  

Density

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.

Summary

• 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.

Review

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 and why? What does the water do?
3. What would need to happen for the all of the oceans to become more saline?

Explore More

Use the resource below to answer the questions that follow.

1. What happens to water as it cools?
2. What plays a crucial role in ocean movement?
3. What does algae require?
4. What do diatoms require?
5. Why is calcium important to organisms in the oceans?
6. Why is phosphate required?
7. How does carbon enter the oceans?
8. What is a dead zone?
9. Where is nitrogen fixed in the ocean?
10. Where does the iron in oceans come from?
11. Why are there plans to seed areas of the ocean with iron?