Introduction: The salinity of a water mass affects the way it interacts with other water masses. 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 varied to increase the differences in density of each water sample. Cold water is denser than warm water.
What to Expect: Students should have some background in density, such as a sink and float activity. 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.
For each pair or group of students:
- Several clear straws
- Food coloring
- Colored Pencils
- Beakers or small test tubes for water
- Four water solutions of different densities, numbered for identification; supply enough for each group of students to test all four:
- Warm (steaming but not boiling) fresh water
- Room temperature fresh water
- Room temperature salty water, 20-35 ppt
- Ice water saturated with salt
1. Introduce the lesson by explaining that ocean water forms density-stratified layers in many places: estuaries, deep bays, continental shelves in summer, and most of the open ocean. 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. Give them time to plan their procedures. If they are unfamiliar with the elements of an investigation, work as a class for a while, then have students break into groups.
2. Have students practice getting the liquid into a straw using a finger over the end of the straw. If necessary, remind students to use only two liquids at a time (limit the variables for each test); encourage experiments to see which color liquid will stack on another liquid.
3. Students can record results by coloring in outlines of the straws on a piece of paper to match the stack or by making diagrams of their own.
4. Have the students repeat procedures 2 & 3 until they have compared all the liquids and recorded results for each pair.
5. Ask students to use the data collected to try stack all four liquids in the straw.
6. Ask students to determine the fewest possible and most steps necessary to find the relative densities of the four liquids.
7. Have students post their results around the classroom. Discuss with students how they decided which liquid was more dense in each trial. Were there any discrepancies?
8. Discuss with students how these results relate to estuaries, polar oceans, and equatorial ocean regions. Where and when are water masses not stratified?
Evaluation: Written communication of results; written or oral explanation of how they compared the different liquids (the methods they used); participation in class discussion.
1. Relate this activity to Mediterranean Sea currents (Count Marsili and the Mediterranean Current) and seasonal turnovers in lakes and ponds (Density Dynamics) .
2. Have students make up solutions of various densities and predict how they will stack, then try them.
3. Have students use reference books and the internet to look into how water masses from different parts of the ocean are tracked as they move around the globe.