Ocean Currents, Part VI: Layering Liquids

 The ocean is made up of layers of water of different densities. Cold water is denser than warm water, water with salt is denser than fresh water. The more closely packed the molecules in a substance, the denser the substance.

This week I challenged them to apply the information that they have learned to create four distinct layers in straw cylinders using only colored water and salt. 

For this experiment you will need: 

• 4 Styrofoam cups or insulated containers
• kosher salt
• plastic straws
• medium raw potato
• medicine droppers
• a package of food coloring

Have students write in their journals a key to the colors:
red-hot and salty
blue-cold and salty
yellow-hot and fresh
green-cold and fresh

 Have four containers to hold water and label them as above, adding about 15 drops of the appropriate food coloring to each container. Fill the containers with water, two with hot water and two with cold. Add 1/3 cup of kosher salt to both the red and blue water. Stir. Add ice to the blue and green containers.

Cut potato into 1'' thick slices. Cut two straws in half for each student. Insert straw into the potato at a 45 degree angle.

 

Sometimes the water will leak out of the bottom. You can ignore it if it is a small amount, or you can take the straw out of the potato over a sink, rinse out the straw and start over again at a new place in the potato, trying to push the straw in deeper.

Have your students decide what order they think the liquids will layer according to density. Have them try out their predictions by adding a tiny bit of the liquids to the straws so that they fill the straw about 1/2 inch, starting with the most dense and adding them one at a time.

 Your students should have layers, starting from the bottom, or the most dense, blue (cold and salty), red (hot and salty), green (cold and fresh), and yellow (hot and fresh).

We got a different result however, in ours. That happens sometimes. In that case, we explore why we got discrepant results. Our layers were, starting with the bottom, or most dense, red (hot and salty), blue (cold and salty), green (cold and fresh), and yellow (hot and fresh). The best explanation we could come up with is that our hot water may have allowed more of the salt to dissolve into the water than the cold. Do you have any other possible explanations?

source: Ocean Currents, Catherine Halversen, Kevin Beals, Craig Strang, Lawrence Hall of Science

Looking at a map, discuss where they think the different types of water might be found.

Cold and salty water is found in Antarctica and the Arctic because the water freezes, leaving the remaining liquid water very salty and cold.

Cold and fresh water is found in places where northern cold rivers drain into the ocean, like in Canada and Siberia.

Warm and salty water is found in areas where the water is warm, but due to the evaporation of water and low river inflow, it is very salty, such as the Mediterranean and Red Seas.

Warm and fresh water is found in places where warm rivers drain into the ocean such as the Amazon and Congo Rivers.

As the waters enter the ocean, they form layers which may last a long time and move long distances.

Sources:

GEMS: Ocean Currents
Grades 5-8
7 Activities
Students gain fascinating insights into our ocean planet through these innovative activities. They learn how wind, temperature, salinity, and density set water into motion, and they make an “in-depth” investigation of the key physical science concept of density. They model how pollution dumped in one location can spread throughout the ocean. Learning is placed in a real-world context as students predict and analyze routes taken by shipwrecked sailors, the 1990 Nike shoe spill, the raft Kon Tiki, and other voyages. In “Message in a Bottle,” students create stories to show what they've learned over the course of the unit.