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Seawater Chemistry

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

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Seawater Chemistry

What do you get when you evaporate seawater?

Salt! Human bodies need salt to live. Nowadays most of this salt comes from the oceans or saline lakes. Some salt comes from rocks that evaporated from seawater.

Makeup of Ocean Water

You know that ocean water is salty. How salty is it? What is the salinity of seawater? Average seawater is about 3.5% salt. The amount is different if there is a nearby source of freshwater. It is also different if there is a lot of evaporation in an area. How do you think these things affect seawater salinity? Seawater also contains other substances like sugars, acids, bases, and organic molecules.

Why Is Ocean Water Salty?

Ocean water is salty because water dissolves minerals out of rocks. The ions enter the water. This happens whenever water flows over or through rocks. Much of this water and its minerals end up in the oceans. Minerals dissolved in water form salts. When the water evaporates, it leaves the salts behind. As a result, ocean water is much saltier than other water on Earth.

How Salty Is Ocean Water?

Have you ever gone swimming in the ocean? If you have, then you probably tasted the salts in the water. By mass, salts make up about 3.5% of ocean water. The table below shows the most common minerals in ocean water ( Table below ). The main components are sodium and chloride. Together they form the salt known as sodium chloride. You may know the compound as table salt or the mineral halite.

Element Percent
Oxygen 85.84
Hydrogen 10.82
Chloride 1.94
Sodium 1.08
Magnesium 0.1292
Sulfur 0.091
Calcium 0.04
Potassium 0.04
Bromine 0.0067
Carbon 0.0028

The amount of salts in ocean water varies from place to place. For example, near the mouth of a river, ocean water may be less salty. That’s because river water contains less salt than ocean water. Where the ocean is warm, the water may be more salty. Can you explain why? (Hint: More water evaporates when the water is warm.)


Seawater has lots of salts in it. This increases its density (mass per volume) over fresh water. Temperature and pressure also affect density.

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 concept "Deep Ocean Currents."


  • density : Mass per unit volume.
  • salinity : Measure of the amount of dissolved salt in water; average ocean salinity is 3.5%.


  • Water moving through rock and soil picks up ions. Those ions end up as salts in large bodies of water.
  • Ocean water contains salts and other substances.
  • Water density increases as salinity and pressure increase, or as temperature decreases.


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?


  1. Streams aren't salty, so why is the ocean salty?
  2. Answer and explain the following: If evaporation is high, what happens to seawater density? If freshwater is added to a region, what happens to seawater density? If seawater gets very cold, what happens to its density?
  3. What is salinity?




Mass per unit volume.


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

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