Chemical Composition & Physical Attributes of Fresh, Brackish & Salt Water

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Discover the differences between fresh, brackish, and saltwater. Learn about their chemical and physical features. Afterward, see how well you know the differences by taking a quiz.

Types of Water

Water is the most important thing for life on Earth. The Earth is covered with water - 70% of the surface is blue. The human body is 60% to 70% water. Some have argued that the fact that this percentage is so similar to the surface of the Earth might not be coincidental. Although life on Earth has done very well on land, and three quarters of Earth's species live there (mostly because of how many insects there are), those species all originally evolved from the sea.

It's really important, therefore, to understand how water moves, acts, and effects ecosystems on our planet. Part of that is understanding that water varies in type and quality. Water can be clear or cloudy, oxygenated or not very oxygenated, and it can be fresh or salty. This latter measure has the biggest impact on ecosystems and forms the main types of water that we study.

The three main types of water are freshwater, brackish water, and saltwater. In this lesson, we're going to discuss the physical and chemical attributes of each of these types of water.

Chemical Composition

Freshwater contains less than 0.05% salt, or less than 1% salt by some definitions. Brackish water contains less than 3% salt. And saltwater contains more than 3% salt.

Water is neutral - it is neither an acid nor a base, but the more salt you dissolve in water, the more basic it becomes. Saltwater is therefore the most basic, followed by brackish water, and freshwater is the closest to neutral.

Saltwater is found in the sea.
Saltwater is found in the sea

When it comes to chemical composition beyond simple water and salt, seawater is very similar from place to place, where as freshwater varies a lot depending on the type of soil and rock the water is in contact with. Seawater may contain hundreds of trace elements including metals like lead and mercury. Seawater also contains large quantities of chlorine (twice as much even than salt), with somewhat smaller quantities of magnesium, sulfur, calcium, and potassium. Freshwater contains similar elements, but much less of everything - fresh water is purer. Brackish water is in between, since brackish water is where fresh and saltwater are mixed, especially in estuaries, lakes, and mangroves.

Brakish water is often found in estuaries.
Brakish water is often found in estuaries

Another difference in the chemical composition of types of water is dissolved oxygen. Oxygen dissolves in freshwater more easily than brackish or salt water. Freshwater also gets the opportunity to dissolve oxygen more often, because it can flow fast in small rivers, stirring plenty of oxygen into itself. But temperature is probably the biggest factor when it comes to dissolved oxygen, not the type of water.

Fresh water can be found in mountain streams.
Fresh water can be found in mountain streams

Physical Attributes

Physical attributes of water includes temperature, clarity (otherwise known as turbidity), and motion. The temperature of water has little to do with the type, and more to do with the depth of water. Large bodies of water, like salty seas, maintain the temperature more easily - they stay cooler in summer and warmer in winter. Smaller bodies of water, which can sometimes be found in freshwater areas, fluctuate more easily. This affects what kind of life can live there.

Turbidity or clarity of water varies by water type.
Turbidity or clarity of water varies by water type

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