With an often unexpected outcome from a simple experiment, students can discover the factors that cause and influence thermohaline circulation in our oceans. In two 45-minute class periods, students complete activities where they observe the melting of ice cubes in saltwater and freshwater, using basic materials: clear plastic cups, ice cubes, water, salt, food coloring, and thermometers. There are no prerequisites for this lesson but it is helpful if students are familiar with the concepts of density and buoyancy as well as the salinity of seawater. It is also helpful if students understand that dissolving salt in water will lower the freezing point of water. There are additional follow up investigations that help students appreciate and understand the importance of the ocean's influence on Earth's climate.
Grade Level(s): 6, 7, 8, 9, 10
Computer for Presenter, LCD Projector, Speakers/Headphones, Adobe Flash Player
1 Hour(s) 30 Minute(s)
Keywords: Fluid circulation, Thermohaline Circulation, Density, Buoyancy, oceanography, physical oceanography, iceberg, current, ocean currents, fluid dynamics
Years ago I gave my students four solutions with varying amounts of salt dissolved in water and asked them to figure out a way to rank them in order from least salty to greatest with only one rule, they weren’t allowed to taste the solutions. There were a number of ideas ranging from measuring the salt left after evaporation to density and buoyancy.
One lab group came up with a unique way of solving this problem. They decided that they would float an ice cube in each solution and that the one that melted the ice fastest would be the saltiest solution, next fastest the third most salty and so on. To our surprise the exact opposite result happened. The ice melted fastest in the water that was least salty (which was actually freshwater) and slowest in the solution that was the most salty. After repeating the experiment we observed that as the ice cubes melted in solutions with less salt, there was circulation that did not happen in salt solutions. We learned something, and I thought that this would make a good laboratory activity, and it has.
I have had success in getting my students engaged because of the unexpected outcome to this experiment.
Source and Access Information
Name of Author/Source: Elizabeth Murray
District/Organization of Contributor(s): Massachusettes Institute of Technology
Access Privileges: Public
* Please note that examples of resources are not intended as complete curriculum.