Hot and Salty, Cool and Fresh

Engage

25 Minutes

Begin a ladder of learning about ocean currents, by posing the question: “What do you know about ocean currents?” and ask students to record the date then write silently for 2-3 minutes everything that comes to mind about ocean currents. After 2-3 minutes ask for volunteers or select students to share what they know. Have students draw a line under what they just recorded and reserve the rest of the notebook page for further entries. This is the first rung of the ladder. Tell students that they will revisit the ladder as they progress through the unit and will be able to see what they’ve learned.

Once again, refer back to the maps that the students made in Investigation 1 showing the locations where the rubber bath toys were found. Give students another chance to revise their original hypothesis about the movement of the toys. Remind them that they have been considering things that happen at the surface of the ocean during the last two investigations: wind, weather systems, waves, and tides. Then explain to them that this investigation will focus on another type of ocean motion: deep currents within the oceans.

Explore

60 Minutes

Pose the question: How do salinity and temperature affect ocean currents? Ask students to look back at their density calculations and think about how water would move in the ocean if there were an area of hot water and an area of cold water. What if the water in one part of the ocean was saltier than another? Ask if they can think of places or circumstances that would produce those conditions. Students can share their responses orally and/or write in their science notebooks.

Students in groups of 2-4 will conduct two experiments that use two clear plastic water bottles connected together. They may be connected with a “tornado tube” or “vortex tube” available at science stores or online. If these tubes are not available, duct tape can be used to secure the bottles together.

Demonstrate the use of the bottles, using ordinary tap water: fill one bottle to the top with water, and screw the tornado tube onto the top. Fill the other bottle with water. Cut the rim off of a plastic yogurt lid so that you have a rigid, flat disk. Hold this tightly over the second bottle and carefully invert it onto the first bottle. Carefully pull out the plastic disk, and tighten the tornado tube onto the top bottle. Or, join the two bottles by holding them carefully and tightly wrapping duct tape around them. Very carefully, over a spill-proof surface, lay the bottles on their sides. Be careful not to move them any more than necessary. Tighten the tornado tube or tape if you can, and hold a blank piece of white paper behind the tubes so that it will be easier to see what’s happening.

Then, have students rotate through two stations where they observe what happens when saline water mixes with fresh water, and what happens when cold and hot water mix.

Explain

30 Minutes

Ask students to gather in groups of four (different from lab partners) and come up with a list, in their notebooks, of what they have learned from their investigation. Each group must have data to share with the entire class, backed up with evidence that they collected. Establish a master list of student findings on a white board, overhead, or other.

After students share their data, make sure the following content and vocabulary are addressed:

1. Salinity and temperature create masses of water with different densities.
2. Water with high salinity and cold temperature is the most dense.
3. Gravity has a greater effect on more dense matter. Hence, more dense water will flow below less dense water.
4. Currents are created by density differences.
5. When water with different densities comes in contact, some mixing occurs.

Discuss different scenarios that lead to mixing in the ocean, such as:

1. Water on the surface in a hot climate evaporates and becomes very hot and salty.
2. Icebergs melt and introduce very cold, fresh water into the ocean.
3. A river enters the sea.
4. Salt water freezes to create fresh ice, leaving a higher concentration of salt in the remaining salt water.
5. There is a hard rainstorm over the ocean.

Discussion should lead to questions such as:
What happens if the water is hot (less dense) AND salty (more dense)? What happens if it is cold AND fresh?

Elaborate

45 Minutes

Allow each group to design and conduct a new experiment using the bottles. They should decide on a question that they would like to answer, write a hypothesis, and give reasons.
For example, their question might be: “What would happen if we put salty room temperature water in one bottle and cold fresh water in the other bottle?”
Their hypothesis might be: “I think the water will mix together right away instead of separating.”
Their reason: “Cold water is more dense than lukewarm water, but fresh water is less dense than salt water. The differences might cancel each other out.

Ask the students to write out their procedures and have you look them over before they begin their test. Check for understanding and correct misconceptions as needed. As the experiments progress, roam around to all groups of students and listen to conversation, pose probing questions, and observe. Redirect if needed.

As with the previous experiments, students should record their observations with notes and drawings, and write a conclusion that compare the results with their hypothesis. They may also discuss any problems that they had.

Evaluate

Formative assessment can include examination of the students’ ladder of learning, lab results, and responses in the science notebooks, as well as attention to class discussion and students’ work in groups.

Extend

This activity is followed by Currents from a Cup.