Brackish water- The type of water found where fresh water and salt water mix. The salt content of the water usually varies and the water is considered neither fresh or salt.
Water is the most abundant chemical on the surface of Earth. Because so many substances dissolve in it, liquid water is almost never found in pure form, except in laboratories. Ocean water, for example, is a complex solution of about 96 percent pure water, 3.0 percent NaCl (common salt), and smaller amounts of many other chemicals (Table 1).
The amount of dissolved salts is not constant from place to place or from time to time throughout the ocean. As a consequence, different regions of water within the ocean can have very different properties. Density of ocean water, for example, is strongly affected by water temperature and amount of dissolved salts. As a result, density varies from place to place. As temperature decreases, density increases until it reaches a maximum at 4°C. Below this temperature, the density begins to decrease. Additionally, as more salts dissolve in water, the water becomes more dense. This trend can continue until the water becomes saturated with salts. Each factor contributes to the variation in ocean water from place to place.
One obvious consequence of differing densities in ocean water is layering, which results in fairly distinct layers of ocean water as depth increases. A less obvious consequence is the formation of density currents. At Earth's poles, the water is both very salty and very cold. These conditions make for very dense water. This water sinks and flows towards the equator along the ocean bottom. Water at the equator flows toward the poles near the surface to replace the sinking water, contributing to global circulation patterns.
Colored solutions are used in this activity to represent three common types of water: ocean (salt) water, brackish water (a mixture of salt and fresh), and river (fresh) water. The amount of salt dissolved in each is by no means the only difference among these three kinds of water. The different concentrations of salt lead to differing densities among these types of water. Fluids having different densities tend to form layers.
Since ocean water contains large amounts of common salt (NaCl) and other dissolved materials, ocean water is more dense than river water. Under certain conditions, river water flowing into brackish estuaries can form a separate layer on top of the more dense ocean water.
Currents, wind conditions, water temperature, and other factors, affect the ways fresh and ocean water mix or form discrete layers. In this activity, students will discover the effects of amount of dissolved substances on water density and layering.