Key concepts
Physics
Convection
Temperature
Density

Introduction
You probably know the ocean never really stays still. But did you know there is something called the global “ocean conveyor belt” that moves massive amounts of water from one ocean to another? These water currents are essential for mixing and transporting nutrients and oxygen and play a critical role in our climate. This is because they move warm and cold water over very long distances, which affects the temperature of the landmasses that border the ocean. The Gulf Stream, for example, is a large current flowing from the Gulf of Mexico that moves warm water into the northern Atlantic Ocean and makes Europe’s climate much milder. Do you know what causes the “ocean conveyer belt”? It is pretty simple: differences in water density. Try this activity to learn more!

Background
There is a lot of water movement in the ocean. The most obvious examples are the waves and ripples on the water's surface that are generated by wind or the ocean currents that are due to tides. It turns out, however, that water can also be moved without wind or tides, which is what happens in the deep ocean. There currents are set in motion by variations in water density caused by differences in temperature and salinity, a process called convection.

The density of a substance is defined by its mass per volume; one liter of water, for example, does not weigh the same as one liter of oil. The lighter liquid floats on the heavier liquid, which sinks to the bottom. Water’s density varies with temperature; when heated it will expand and therefore increase in volume. This means its mass per volume will decrease and it becomes less dense. Warm water, thus, is generally less dense than cold water. Another factor that affects water density is salinity, which is the amount of salt in ocean water. More salt makes water heavier and therefore denser.

These two factors, temperature and salinity, are the main driving forces behind Earth’s ocean conveyor belt, which is a huge water circulation system in the deep ocean that moves water around the globe. Currents begin near the North Pole in the North Atlantic where the surface of the ocean gets cooled by the arctic temperatures. As sea ice forms, salt stays behind and the water there becomes saltier. This water is now denser and will sink to the bottom of the ocean floor. This creates a current as surface water has to move in to replace the sinking water. The deep, cold water moves all the way to Antarctica and then to the Indian and Pacific Oceans. Once the water reaches warmer regions it heats up, becomes less dense and rises toward the surface. It eventually finds its way back to the North Atlantic where the whole cycle begins again. In this activity you will demonstrate how temperature affects water movement and water density—and create your own little ocean currents in a glass!

Materials

• Two transparent cups, exactly the same size
• An old CD
• Sandwich bag
• Scissors
• Food coloring (any color)
• Tap water
• Microwave oven
• Ice cubes
• Thermometer
• Paper towels for water spills
• Tray
• Table salt (optional)

Preparation

• Use the scissors to cut a long strip off the sandwich bag, wide enough to fully cover the hole in the middle of the CD.
• Place the tray on a work area that can tolerate water spills.
• Fill both cups with tap water.
• Add ice cubes to one of the cups to cool down the water. Once it is cold, remove the ice cubes and fill the cup to the brim with more tap water.
• Put the other cup into the microwave for about one minute. (If it is too hot, let it cool down until you can pick up the cup.)
• Make sure it is filled to the brim with water.
• Add a couple of drops of food coloring to the hot water.
• Set the cup with hot water into the tray.
• With the thermometer, measure the temperature of both the hot and cold water.

Procedure

• Place the cut plastic strip from the sandwich bag onto the CD so the hole is completely blocked, and part of the plastic strip extends beyond the CD.
• Put the CD on top of the cold cup with the plastic strip facing toward the cold water.
• Then pick up the cup with cold water and the CD on top and slowly turn it upside-down, pressing on the CD to avoid any spillage. You might need an adult to help you with this step.
• Place the upside-down cup with cold water on top of the cup with hot water so the CD is separating both cups. Once you have placed both cups together, what happens? Do you see any water mixing yet?
• Slowly pull on the part of the plastic strip that extends beyond the CD and remove the whole strip to expose the CD’s hole. What happens when the hole is open? Do you see any water movement? If yes, in which direction does the water move? Can you describe your observations?
• Observe both cups for about 10 minutes. Do you see any changes happening in the top or bottom cup? If yes, how does the water change?
• After 10 minutes carefully separate both cups again by lifting the CD and the upper cup and inverting both again. You might want to ask an adult to help you with that.
• Finally, measure the temperature again in both cups. Has the temperature changed? If yes, how?
• Drain the water in both cups into the sink and set up the same test again, but this time place the cup with hot water on top of the cup with cold water. (You should have an adult to help you with this.) What happens if you remove the plastic strip this time? Do you see the same results? Does any water movement happen? Why or why not?
• After 10 minutes, separate both cups again and measure the temperature in each cup. Has the temperature changed? If yes, how much?
• Extra: In this activity you have shown differences in temperature drive water movement. What happens if you have no temperature gradient? Repeat the same test, but this time do not heat or cool the water in the cups—keep them at the same temperature. Do you see any water movement happening when you remove the plastic strip? Why or why not?
• Extra: Can you find a way to demonstrate how salinity affects water density and water movement? Add salt to one of the cups to create a salinity gradient and repeat the test. Does the salty water float or sink? Can you create water movement with a salt gradient?
• Extra: Instead of demonstrating that warm water floats atop cold water, show how cold water sinks to the bottom and add the food coloring to the cold water instead of the warm water. Then place the cold water on top of the warm water and vice versa. Do you see water movement in both directions, or only in one?
• Extra: How does the temperature gradient affect the water mixing? Add a third test in which the temperature difference between the warm and cold water is different from your previous test. What changes if you have a higher versus a lower temperature difference between the hot and cold water?

Observations and results
Were you able to make your water move to float and sink? When you add the food coloring to the hot water and place the cold water on top of the hot water, initially nothing should have happened as the plastic strip should have still covered the hole of the CD, preventing the water from mixing between the cups. As soon as you remove the plastic strips, however, the dyed warm water will shoot through the CD hole and mix with the cold water above.

This water movement is driven by convection: The movement of water due to differences in temperature. Because the warm water is lighter than the cold water and wants to expand, it moves into the upper cup through the hole. After about 10 minutes the mixing should almost be completed, and the color of the water should be similar in both cups. At the same time the temperature in both cups should have changed significantly. In the beginning you should have measured a large temperature difference. But at the end, both temperatures should have been similar. As the water was mixing heat exchange occurred, which means the warm water heated the cold water. If you tried the extra activity with a higher or lower temperature difference between both cups, you should have noticed the mixing of water happens faster when the temperature difference is larger. When you have no temperature gradient at all, no significant mixing will happen within 10 minutes.

If you reversed the test and have the warm on top of the cold water, you should have observed the water did not mix between the cups. This is because warm water is lighter, or less dense, than the cold water, which is why it wants to remain on top. The same is true for salty versus non-salty water. Salty water will always sink to the bottom, as it is denser than nonsalty water—exactly how it does in the real ocean!

Cleanup
Drain all the water in the sink and clean any spills.

More to explore
The Great Ocean Currents—The Climate Engine, from World Ocean Review
The Ocean—a Driving Force for Weather and Climate, from NASA
Ocean Currents: Modeling the “Global Conveyor Belt” in Your Kitchen, from Science Buddies
High Seas: What Happens When the Glaciers Melt?, from Scientific American
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies