Students love snow… and snow days! Last week, we certainly saw enough of our favorite type of winter weather to capture their attention. This week, why not try to focus that energy to teach about times when the bay has frozen over?
Did you even know that it is possible for the Chesapeake Bay to freeze over? It’s true! During the winter of 1779-1780 ice in the bay was so thick that carriages could be driven over it from Annapolis to Poplar Island. The winter of 1976-1977 was similarly severe. Ice on the bay was so solid that the National Guard was mobilized to deliver food and supplies to stranded residents on Smith and Tangier Islands.
Talking about Chesapeake Bay freeze-ups can help teach students about estuaries and salinity. In an estuary, such as the Chesapeake Bay, tributary rivers deliver freshwater that mixes with saltwater from the ocean. This mixture of freshwater and saltwater is called brackish water. Saltwater, which is denser then freshwater, sinks below the freshwater. Mixing between the two can be influenced by tides, winds, currents, waves, temperature, and the amount of freshwater runoff. Saltwater freezes at a lower temperature then freshwater, and because the Chesapeake Bay is brackish, it rarely freezes over.
It is easy to teach about how waters with different salinities freeze at different temperatures! To do so, all you need is a freezer, tap water, table salt, a thermometer, masking tape, a permanent marker, and six Tupperware containers/beakers. Label two of the beakers “Freshwater,” two “Brackish,” and two “Sea Water.” In the “Sea Water” containers, mix 10 tablespoons of table salt with one cup of water. Mix 5 tablespoons of table salt with one cup of water in the two “Brackish” containers, and add one cup of water with no salt to the two containers labeled “Freshwater.” Have students use the thermometer to check the water temperature in each container after 30 minutes, one hour, 24 hours, and 48 hours. If some ice has formed, try to check the temperature under the water. Have students discuss how and why ice forms more quickly in waters with lower salinity. Ask students where they think ice would be more likely to start forming in the Chesapeake Bay: in tributaries or at the bay’s mouth.
Check out the following resources for more information on Chesapeake Bay freeze-ups and for similar lesson plans on salinity:
- Density and Salinity – This resource from the UCLA Marine Science Center includes multiple activity lessons addressing sea ice, icebergs, conductivity, making and using a hydrometer, and the layering of water.
- Can Sea Water Freeze? – In this lesson from McREL, students will learn about the unique properties of sea water, including that its freezing point is slightly lower than that of freshwater.
- Can Sea Water Freeze? – NASA’s Aquarius provides this lesson about how salt and other substances impact the freezing point of water.