Salinity

If you have ever gone swimming in the ocean, you probably did not notice that it was easier to swim than in freshwater. But if you tried to swim in the great Salt Lake, you probably had some difficulty swimming below the surface of the water. This is because sea water contains many dissolved materials including NaCl.

The reason people can swim in freshwater is because water is 775 times more dense than air. As a result, the density of water makes us relatively buoyant against gravitational pull. As the concentration of dissolved materials increases, the density of water increases in approximately a linear fashion. At 4^oC, the density of pure water is 1.0000; at 35 parts per thousand salinity, it is 1.20822.

• What is the salinity and density of the Great Salt Lake?

The salinity of freshwater is usually less than 0.1 g/L. For sea water, that is 100% sea water, salinity is about 35 parts per thousand. If water only contained NaCl,

• how many grams of NaCl are found in 100% sea water?
• how many grams of NaCl are found in 20% sea water?
• what is the percent of NaCl found in freshwater?

Salinity decreases the temperature at which water is most dense. The temperature of maximum water density decreases about 0.2^oC for every 1.0 g/L of dissolved materials.

• If the maximum density of freshwater is at 4^oC, then at what approximate water temperature would a 50% sea water be most dense?

Salinity also depresses freezing temperature. One hundred percent sea water freezes at -1.91^oC.

• Why does salt, or any dissolved material, reduce the freezing point of water?

Salinity is a factor in water pollution as well. When freshwater is drawn from aquifers, it may allow the intrusion of salt water which will float on top of freshwater. The result is salty drinking water. You have probably noticed that when you spill something on a counter and do not wipe it up, that a deposit is left when the water evaporates. A result of irrigation farming is the eventual increase in salt concentration of the land because water evaporates depositing the salts it contains. If a body of water receives input, but no real output other than evaporation, the concentration of salts in this body of water will also increase. This is what happens in the Great Salt Lake and coastal ponds in Bahrein. 

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