Seawater is a complex mixture that constitutes the oceans and seas , covering over 70% of the Earth's surface. It is composed of approximately 96.5% water, 2.5% salts, and smaller amounts of other substances, including dissolved inorganic and organic materials, particulates, and a few atmospheric gases. The six most abundant ions in seawater are chloride, sodium, sulfate, magnesium, calcium, and potassium, making up about 99% of all sea salts by weight. Salinity, which ranges from 34 to 37 parts per thousand, indicates the salt content in seawater. Seawater also contains dissolved atmospheric gases like nitrogen, oxygen, argon, and carbon dioxide, as well as dissolved organic substances like carbohydrates and amino acids.

Where is seawater found?

Seawater is found in the oceans and seas that cover nearly 71% of Earth's surface. Oceans are vast, unconfined bodies of saltwater, while seas are smaller, shallower, and often partially enclosed by land. The Atlantic , Pacific , Indian , Arctic , and Southern are the major oceans, separated by arbitrary boundaries, largely defined by the continents that frame them. Seawater salinity ranges from 34 to 37 parts per thousand.

What makes seawater different from freshwater?

Seawater is distinct from freshwater primarily due to its salinity, or salt content. Seawater has an average salinity of 35 parts per thousand (psu), meaning it contains about 35 grams of salt per kilogram. In contrast, freshwater has a salt concentration usually less than 1 psu. The six most abundant ions in seawater are chloride, sodium, sulfate, magnesium, calcium, and potassium, constituting about 99% of sea salts. This higher salt content results in seawater having a lower freezing point, a higher boiling point, greater density, and greater viscosity than freshwater.

What is salinity and why is it important in seawater?

Salinity refers to the amount of dissolved salts in water. It is commonly measured in practical salinity units (psu) or parts per thousand. Open ocean salinity ranges from about 34 to 37 psu. Salinity impacts several seawater properties, including density, freezing point, and viscosity. Salinity, along with temperature, is used to calculate seawater density. Also, it influences thermohaline circulation , which drives the vertical circulation of oceans by replacing surface water with water from the depths. Moreover, salinity is vital for marine organisms, with tolerance varying among species. For example, stenohaline organisms have low tolerance to salinity changes, while euryhaline organisms can tolerate large changes in salinity.

What substances are dissolved in seawater besides salt?

Seawater contains a variety of dissolved substances in addition to salt. The six most abundant ions are chloride, sodium, sulfate, magnesium, calcium, and potassium. Other major dissolved substances include inorganic carbon, bromide, boron, strontium, and fluoride. Seawater also contains dissolved atmospheric gases like nitrogen, oxygen, argon, and carbon dioxide, as well as dissolved organic substances such as carbohydrates and amino acids. Additionally, it contains minor dissolved chemical constituents, including inorganic phosphorus and inorganic nitrogen, which are crucial for the growth of marine organisms.

How does seawater influence Earth's climate and weather?

Seawater significantly influences Earth's climate and weather patterns through heat and moisture exchange with the atmosphere. Oceans absorb and store heat from the atmosphere, acting as a heat reservoir, and ocean currents redistribute heat globally. For example, the Gulf Stream warms Northern Europe, and the Peru (Humboldt) Current cools South America. Evaporation from the ocean adds moisture to the atmosphere, which leads to cloud formation and precipitation. The oceans also drive weather systems by influencing the position of the jet stream. Anomalies in sea surface temperatures, such as El Niño, can cause major climate changes across the globe.

seawater

Introduction
Chemical and physical properties of seawater
- Chemical composition
  - Dissolved inorganic substances
  - Dissolved organic substances
  - Effects of human activities
- Salinity distribution
- Temperature distribution
- Thermal properties
- Density of seawater and pressure
- Optical properties
- Acoustic properties
Chemical evolution of seawater
- The early oceans
- The transition stage
- The modern oceans

References & Edit History Quick Facts & Related Topics

Images & Videos

Side-by-side diagram showing ocean acidification from the late 1800s to a projection for 2100. The left panel illustrates lower atmospheric carbon dioxide, higher seawater pH, and abundant healthy corals and shells due to more carbonate ions. The right panel shows higher carbon dioxide, lower pH, fewer carbonate ions, and fewer, smaller marine calcifiers, highlighting the negative impact on ocean life.

Diagram showing three stages of saltwater intrusion into a coastal aquifer: the first panel shows normal conditions with a stable interface between ocean saltwater and fresh groundwater; the second shows increased saltwater intrusion and a cone of depression when groundwater is pumped; the third shows how an injection well restores pressure and blocks saltwater intrusion. Labels identify ocean, aquifer, potentiometric surface, and water flow directions.

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At a Glance

seawater summary

seawater

oceanography

Also known as: salt water, saltwater

Written by

Alyn C. DuxburyAll

Fact-checked by

Britannica Editors

Last updated

Oct. 8, 2026 •History

Top Questions

What is seawater?
Seawater is a complex mixture that constitutes the oceans and seas, covering over 70% of the Earth's surface. It is composed of approximately 96.5% water, 2.5% salts, and smaller amounts of other substances, including dissolved inorganic and organic materials, particulates, and a few atmospheric gases.

The six most abundant ions in seawater are chloride, sodium, sulfate, magnesium, calcium, and potassium, making up about 99% of all sea salts by weight. Salinity, which ranges from 34 to 37 parts per thousand, indicates the salt content in seawater. Seawater also contains dissolved atmospheric gases like nitrogen, oxygen, argon, and carbon dioxide, as well as dissolved organic substances like carbohydrates and amino acids.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where is seawater found?
Seawater is found in the oceans and seas that cover nearly 71% of Earth's surface. Oceans are vast, unconfined bodies of saltwater, while seas are smaller, shallower, and often partially enclosed by land. The Atlantic, Pacific, Indian, Arctic, and Southern are the major oceans, separated by arbitrary boundaries, largely defined by the continents that frame them. Seawater salinity ranges from 34 to 37 parts per thousand.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What makes seawater different from freshwater?
Seawater is distinct from freshwater primarily due to its salinity, or salt content. Seawater has an average salinity of 35 parts per thousand (psu), meaning it contains about 35 grams of salt per kilogram. In contrast, freshwater has a salt concentration usually less than 1 psu. The six most abundant ions in seawater are chloride, sodium, sulfate, magnesium, calcium, and potassium, constituting about 99% of sea salts. This higher salt content results in seawater having a lower freezing point, a higher boiling point, greater density, and greater viscosity than freshwater.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What is salinity and why is it important in seawater?
Salinity refers to the amount of dissolved salts in water. It is commonly measured in practical salinity units (psu) or parts per thousand. Open ocean salinity ranges from about 34 to 37 psu.

Salinity impacts several seawater properties, including density, freezing point, and viscosity. Salinity, along with temperature, is used to calculate seawater density. Also, it influences thermohaline circulation, which drives the vertical circulation of oceans by replacing surface water with water from the depths.

Moreover, salinity is vital for marine organisms, with tolerance varying among species. For example, stenohaline organisms have low tolerance to salinity changes, while euryhaline organisms can tolerate large changes in salinity.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What substances are dissolved in seawater besides salt?
Seawater contains a variety of dissolved substances in addition to salt. The six most abundant ions are chloride, sodium, sulfate, magnesium, calcium, and potassium. Other major dissolved substances include inorganic carbon, bromide, boron, strontium, and fluoride.

Seawater also contains dissolved atmospheric gases like nitrogen, oxygen, argon, and carbon dioxide, as well as dissolved organic substances such as carbohydrates and amino acids. Additionally, it contains minor dissolved chemical constituents, including inorganic phosphorus and inorganic nitrogen, which are crucial for the growth of marine organisms.
2 Britannica Sources
1. seawater: Dissolved organic substances
2. salt: Seawater
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does seawater influence Earth's climate and weather?
Seawater significantly influences Earth's climate and weather patterns through heat and moisture exchange with the atmosphere. Oceans absorb and store heat from the atmosphere, acting as a heat reservoir, and ocean currents redistribute heat globally. For example, the Gulf Stream warms Northern Europe, and the Peru (Humboldt) Current cools South America.

Evaporation from the ocean adds moisture to the atmosphere, which leads to cloud formation and precipitation. The oceans also drive weather systems by influencing the position of the jet stream. Anomalies in sea surface temperatures, such as El Niño, can cause major climate changes across the globe.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

seawater, water that makes up the oceans and seas, covering more than 70 percent of Earth’s surface. Seawater is a complex mixture of 96.5 percent water, 2.5 percent salts, and smaller amounts of other substances, including dissolved inorganic and organic materials, particulates, and a few atmospheric gases.

Seawater constitutes a rich source of various commercially important chemical elements. Much of the world’s magnesium is recovered from seawater, as are large quantities of bromine. In certain parts of the world, sodium chloride (table salt) is still obtained by evaporating seawater. In addition, water from the sea, when desalted, can furnish a limitless supply of drinking water. Many large desalination plants have been built in dry areas along seacoasts in the Middle East and elsewhere to relieve shortages of fresh water.

Chemical and physical properties of seawater

The six most abundant ions of seawater are chloride (Cl⁻), sodium (Na⁺), sulfate (SO²₄⁻), magnesium (Mg²⁺), calcium (Ca²⁺), and potassium (K⁺). By weight these ions make up about 99 percent of all sea salts. The amount of these salts in a volume of seawater varies because of the addition or removal of water locally (e.g., through precipitation and evaporation). The salt content in seawater is indicated by salinity (S), which is defined as the amount of salt in grams dissolved in one kilogram of seawater and expressed in parts per thousand. Salinities in the open ocean have been observed to range from about 34 to 37 parts per thousand (0/00 or ppt), which may also be expressed as 34 to 37 practical salinity units (psu). See also salinity.

Inorganic carbon, bromide, boron, strontium, and fluoride constitute the other major dissolved substances of seawater. Of the many minor dissolved chemical constituents, inorganic phosphorus and inorganic nitrogen are among the most notable, since they are important for the growth of organisms that inhabit the oceans and seas. Seawater also contains various dissolved atmospheric gases, chiefly nitrogen, oxygen, argon, and carbon dioxide. Some other components of seawater are dissolved organic substances, such as carbohydrates and amino acids, and organic-rich particulates. These materials originate primarily in the upper 100 metres (330 feet) of the ocean, where dissolved inorganic carbon is transformed by photosynthesis into organic matter.

Many of the characteristics of seawater correspond to those of water in general, because of their common chemical and physical properties. For example, the molecular structure of seawater, like that of fresh water, favours the formation of bonds among molecules. Some of the distinctive qualities of seawater are attributable to its salt content. The viscosity (i.e., internal resistance to flow) of seawater, for example, is higher than that of fresh water because of its higher salinity. The density of seawater also is higher for the same reason. Seawater’s freezing point is lower than that of pure water, and its boiling point is higher.

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Chemical composition

The chemical composition of seawater is influenced by a wide variety of chemical transport mechanisms. Rivers add dissolved and particulate chemicals to the oceanic margins. Wind-borne particulates are carried to mid-ocean regions thousands of kilometres from their continental source areas. Hydrothermal solutions that have circulated through crustal materials beneath the seafloor add both dissolved and particulate materials to the deep ocean. Organisms in the upper ocean convert dissolved materials to solids, which eventually settle to greater oceanic depths. Particulates in transit to the seafloor, as well as materials both on and within the seafloor, undergo chemical exchange with surrounding solutions. Through these local and regional chemical input and removal mechanisms, each element in the oceans tends to exhibit spatial and temporal concentration variations. Physical mixing in the oceans (thermohaline and wind-driven circulation) tends to homogenize the chemical composition of seawater. The opposing influences of physical mixing and of biogeochemical input and removal mechanisms result in a substantial variety of chemical distributions in the oceans.

seawater

Chemical and physical properties of seawater

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Chemical composition