Prerequisite: It is important to familiarize the students with a look at the geography of the Mediterranean region. It would also be beneficial if your students are familiar with currents and density, although neither is necessary.
Introduction: Ancient sailors knew that the Mediterranean Sea had very strong currents flowing into it from the Black Sea and the Atlantic Ocean. The puzzle was that the water level never seemed to rise even though there was no apparent exit for this excess water. Many theories were introduced including the existence of under water channels. In 1679, Count Luigi Marsili solved this puzzle by setting up a model of the Mediterranean. He demonstrated the current flow using water masses of different salinities. Water in the Mediterranean has a high salinity and thus is denser than the Atlantic and the Black Sea’s Water. Current leaving the Mediterranean flows below the incoming current.
What to Expect: This demonstration requires considerable set-up time but is dramatic and memorable.
- 150 ml of salt
- Aquarium or clear plastic box
- Aluminum Foil (for a more permanent demonstration set-up, use plexiglas, sturdy plastic or a thin panel of wood to make the barrier, fastening it in place with waterproof caulk. Make the holes ahead of time and then follow the procedure given here)
- Masking Tape
- Food Coloring
- Black Pepper
- (2) 1-Liter Containers, or increase the volumes of water and salt in proportions given here if you are using a large tank
1. Prior to the lab, share the background described in the introduction and ask the students: What may be the reasons for these strange currents? Why does the water level not rise in the Mediterranean? How would solving this puzzle help the people of the time?
2. Cut a piece of aluminum foil (for alternatives, see materials) slightly larger and wider than the aquarium. Tape it into the aquarium so that it makes a waterproof barrier that divides the aquarium in two.
3. In one container mix 1000 ml of water with 150 ml of salt and a few drops of red food coloring. This represents the salty Mediterranean.
4. Fill the second container with 1000 ml of water and add blue food coloring. This represents the less salty Atlantic.
5. At the same time, pour the contents of one container into one side of the aquarium and the contents of the second container into the other side of the aquarium so that the aluminum foil barrier separates the two. Make sure to pour them at the same time, otherwise the barrier will collapse.
6. Sprinkle pepper on the blue side of the aquarium (the Atlantic side).
7. Puncture one hole in the barrier just below the surface of the water and one hole near the bottom of the aquarium. Each hole should be approximately 1 cm in diameter.
8. Observe for approximately 10 minutes.
Discuss the following questions with the students:
1. What was the current flow that you saw in the lab?
2. From your observations, which was saltier, the Atlantic or the Mediterranean? How can you tell?
3. Why does salinity differ between the bodies of water?
Student participation in discussion; lab worksheets outlining their group=s test of the water masses; conclusions and discussions.
Extensions: Individual Tests of the Theory
Challenge students to test Marsili’s model. Their assignment will be to demonstrate how two water masses of different salinities will interact. Advise them to add the water to the containers slowly, pouring it down the side of the container. Make sure they write out the steps they take to obtain their results so other groups can duplicate their efforts, and be sure they have clear conclusions and discussion sections. Tell them they will be provided with water from different depths in the Straits of Gibraltar. (This is where it pays to be an actor. The more you ham it up about how you got this water, the more fun your kids will have). The water recipe is:
- 50 ml of salt to 1000 ml of water (at room temperature) from the middle layer
- 150 ml salt to 1000 ml of H20 (cool with ice to increase density)from the bottom layer
- plain tap water (warm this slightly above room temperature) from the top layer. Note: Atlantic ocean water is salty, of course, but using fresh water will result in more easily observed differences
Have students use food coloring to keep track of the water masses. Provide them with a variety of test tubes, beakers, droppers, graduated cylinders, and measuring devices. Students will need to determine the problem, hypothesis, experiment, observation recording method, and conclusion when they have finished their lab. If your students have never done this sort of lab, you may want to do some or all of it as a class.