Exploring Our Fluid Earth

Seawater is a Solution

Fig. 2.2.  (A) Pure water consists of only hydrogen and oxygen combined into water molecules (H₂O). (B) Seawater is a mixture of pure water and dissolved ionic substances.

Image by Byron Inouye

Water is a very good solvent. Solvents are liquids that dissolve other substances. Most of the water on earth, including the water in oceans, lakes, rivers, and ponds, contains many solutes. Solutes are substances that are dissolved in a solvent, like elements and compounds. A solution is a mixture of a solvent and solutes. In the solution of seawater, water is the solvent. The hydrogen and oxygen atoms in water molecules make up about 96.5 percent of the mass of seawater. This means that in a seawater solution, about 3.5 percent of the mass is made up of dissolved solutes like Na⁺ and Cl^– (Fig. 2.2).

Sources of Dissolved Substances in Seawater

Seawater is a dynamic chemical mixture that interacts constantly with the land, the atmosphere, and living things. When rainwater flows over the land, it dissolves substances from soil and rocks. Runoff carries these materials directly into the ocean or into streams and rivers that empty into the ocean (Fig. 2.3 A). Rain falling into the ocean carries gases and small particles of soot and dust. Atmospheric gases mix and dissolve into seawater, especially when winds and waves churn the ocean surface (Fig. 2.3 B).

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Fig. 2.3. (A) Aerial photo showing river runoff from the La Plata River in South America mixing into the ocean.

Image A courtesy of UNESCO via U.S. Geological Survey

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Fig. 2.3. (B) The ocean interacting with the atmosphere via sea spray at the Halona blowhole on O‘ahu.

Image by Alyssa Gundersen

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Fig. 2.3. (C) A black smoker (Sully Vent) covered with tube worms in the Main Endeavor Vent Field in the northeast Pacific ocean basin. An acoustic hydrophone and resistivity-temperature-hydrogen (RTH) probe rests on the vent.

Image courtesy of National Oceanic and Atmospheric Association (NOAA) Pacific Marine Environmental Laboratory (PMEL) Earth-Ocean Interactions (EOI) Program

Seawater also dissolves materials from the ocean bottom, as well as materials released by underwater volcanoes and hydrothermal vents (Fig. 2.3 C). The constant addition of dissolved substances into the ocean over billions of years has made the ocean salty.

Salts in Seawater

Salt is a common substance that comes in many forms, including table salt (Fig. 2.4 A), rock salt (Fig. 2.4 B) and sea salt.

Fig. 2.4. (A) table salt

Image courtesy of Edal Anton Lefterov

Fig. 2.4. (B) rock salt

Image courtesy of Lordtct

When seawater evaporates, sea salt is left behind. If seawater evaporates from a surface with a slight curve, such as a watch glass or a shallow tidepool, the salt forms distinct rings (e.g. Fig. 2.5 A). A close-up of these rings on a watch glass is shown in Fig. 2.5 B. The rings are made up of different types of salts in the mixture of seawater. Each ring is composed of salts with different chemical compositions and properties.

Fig. 2.5. (A) Rings of salt from seawater evaporated on a watch glass.

Image by Joanna Philippoff

Fig. 2.5. (B) A close-up of salt rings on watch glass.

Image by Joanna Philippoff

When seawater evaporates, sea salt is left behind. If seawater evaporates from a surface with a slight curve, such as a watch glass or a shallow tidepool, the salt forms distinct rings (Fig. 2.5 A). A close-up of these rings on a watch glass is shown in Fig. 2.5 B. The rings are made up of the different types of salts in the mixture of seawater. Each ring is composed of salts with different chemical compositions and properties.