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8.11: Seawater Chemistry

Difficulty Level: At Grade Created by: CK-12
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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.

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?

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!

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. Check out the video below:


Interactive ocean maps can show salinity, temperature, nutrients, and other characteristics: http://earthguide.ucsd.edu/earthguide/diagrams/levitus/index.html.


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 Currents" lesson.


  • 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.

Making Connections


Use this resource to answer the questions that follow.

Ocean Chemistry


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 problems does increased ocean acidity cause?

9. What is a dead zone?

10. Where is nitrogen fixed in the ocean?

11. Where does the iron in oceans come from?

12. Why are there plans to seed areas of the ocean with iron?


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?

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A measure of the amount of dissolved salt in water; average ocean salinity is 3.5%.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 07, 2016
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