Case Study: Hot, Cold, Fresh and Salty

Miss Sherwood wanted to review convection with her 9th grade earth science class. She had already taught convection in astronomy, meteorology, and geology. Reviewing convection made it really easy to demonstrate how temperature and density create ocean currents.

Using the online resources under "The Ocean's Role in Weather and Climate," she found an activity called “Hot, Cold, Fresh, Salty.”  This activity was designed to demonstrate how temperature creates currents as well as how different densities create currents. Miss Sherwood used the worksheet that came with the activity. She did add two images – surface currents and the ocean conveyor system – to the worksheet so student could see the different types of currents. 

The original activity called for small groups to complete the activity. Unfortunately, Miss Sherwood's class had to do the activity as a large group because many of the students are very clumsy and we have had major clean-ups after other liquid labs. Different solutions of salt water were created ahead of time due to shortened class times and different temperature solutions were created by storing solutions in the fridge or heating them during the experiment in a microwave using microwave-safe containers. Students themselves poured the solutions into the shoeboxes taking turns as they completed different steps and discussed what happened.

The students remembered convection in turns of hot and cold and knew that the cold blue water would sink to the bottom and the hot red water would rise to the top. Their experiment supported their hypothesis. They also remembered how denser substances sink and predicted that the red hypersaline solution would sink to the bottom while the blue hyposaline solution would rise to the top. Once again their experiment supported their hypothesis.

To further demonstrate temperature differences, students created four different temperature solutions of different colors. When poured into the shoeboxes, momentary differences could be seen, but mixing occurred to quickly. For future experiments, Miss Sherwood wants to try funnels to see if her class can control where the solutions end up.

For the final step of determining which was more important, temperature or salinity, Miss Sherwood had individuals think of different options and then discuss it with the person next to them. Volunteers then described their experiment and the class tried three of them.  One group suggested trying to pour 2 hypersaline solutions of very hot and very cold. They mixed. A second group hypothesized using very cold water of hyposaline and hypersaline solutions. One again they mixed, but the blue hypersaline solution originally was on the bottom. Going from previous experiments, a final group hypothesized that hot hypersaline solution and a cold hyposaline solution would show which was more important. Their experiment worked and showed that salinity was more important since hot high saline water ended up below the colder low saline solution.

The NOS Education resources listed several Web sites that further explained the material for both teachers and students. Ocean World:  What is a Current? is an excellent resource from TAMU and goes through many of the different types of currents and how they are formed. The Thermohaline Circulation website had a great animation showing the different parts of the ocean conveyor system. 

Miss Sherwood looks forward to trying this experiment in the beginning of the year as a way to actually teach the concept since many students said that it helped to clarify convection.

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