As the salinity of water increases, so does its density. In estuaries, where fresh water flowing from rivers meets salt water from the ocean, the fresh water tends to float above the salt water until mixed by wind or tidal currents. This affects the distribution of both organisms and sediments. In this demonstration, temperature is also v aried to increase the differences in density of each water sample.
Cold water is denser than warm water.
It is generally helpful to introduce the concept of density using solids before working with liquids. This lesson is an ideal time to have students work on an organized approach to problem-solving. They should be given time to plan how to determine the relative densities of the liquids and should have data sheets ready for use before they begin. Be sure to allow time for students to test questions of their own which come up over the course of this activity. Direct them to use the straws, food coloring, and samples to design and carry out a series of tests to show how water masses of four different densities interact.