Test Density
So what are salinity and density?
Before you begin, it is necessary to understand two important differences between freshwater and seawater. One is salinity (suh-LIN-ih-tee), which means how salty a solution is. As you'd probably guess, ocean water has greater salinity than freshwater. Another important difference between freshwater and seawater is their density. Density refers to the amount of "stuff" in a given space or packed in a space. For example, a 10 inch round pan containing a fluffy angel food cake would have a lower density than the same size pan filled with a fudge cake. The salinity of water affects its density. The saltier water is, the denser it is. Which is denser — a cup of ocean water or a cup of freshwater? To help you answer this question, look at the pictures to the right.
 
Freshwater is made up of mostly water molecules with a little bit of salt dissolved among the molecules. Ocean water is made up of water molecules and a lot of salt dissolved among the water molecules. When put in the same container, denser materials sink to the bottom and less dense materials float to the top.