Conductivity of Freshwater vs. Seawater

Purpose

To compare the conductivity of freshwater and seawater. This is a lab activity, and students should be taught appropriate safety measures, including eye protection and safety while using electricity.

Procedure

1. Use the table in attached PDF to record your data.
2. Put 100 mL of 3.5% NaCl solution into a 250 mL beaker.
3. Attach 2 electrodes, one to each side of a copper plate (other metals may be used)
4. Attach the other end of the 2 electrodes to an ammeter.
5. Record the current in amps, in the chart above.
6. Unhook the apparatus and discard the solution.
7. Draw a labelled diagram of the apparatus.
8. Repeat steps 1-5, using the remaining 3 solutions.
9. Clean your lab station and return the apparatus to its original location
   Note: Caution: use of electricity, and solutions that may irritate the eyes.

Discussion

1. Make a line graph of your data, labelling the x-axis "Concentration (%)" and the y-axis "Conductivity (Amps)".
   Use a Ruler. Remember to give your graph a title. Salt Concentration vs. Time.

2. What is the relationship between salt concentration and conductivity?
   The greater the salt concentration, the greater the conductivity.
   The slope should be positive and to the right.

3. Why is seawater conductivity useful in studying marine ecosystems?
   Marine plants and animals require specific levels of salinity to survive. Therefore, a change in conductivity could be an indication of threats to marine animals and plants.

4. What affects the concentration of salt in oceans?
   Salt concentration can be affected by melting ice, evaporation, rain, snow, wind waves, ocean currents and inflow of fresh water from rivers.

5. Why do the Earth’s oceans maintain a fairly constant salt water concentration?
   The removal of salts from the ocean is balanced by the “input” of salt. For example, the removal of salts could occur from the deposits of salt left after seawater evaporates, or the formation of sediments, such as calcium carbonate. The balance is maintained because the removal of salts increases with increased concentration.

6. Do you think the conductivity in the Polar oceans will change as sea ice melts? Why?
   As sea ice (which is salt water ice) melts, the ocean’s salinity won’t change. Therefore the conductivity will not change.

7. Is ice from land salty?
   No.

8. a) Do you think the conductivity in the Polar oceans will change as Polar ice shelves melt?
   Yes, the conductivity may decrease in the areas that receive direct ice melt, as it may dilute the water.
   b) How might marine ecosystems (both plant and animal) be affected?
   Marine ecosystems that depend on a certain level of salinity will either move or die. As ice shelves melt, the feeding patterns and migration patterns of the animals that hunted in those areas change, thereby affecting Arctic hunters. Freshwater ecosystems from the ice shelf will die as the freshwater melts into the ocean.
   As ice melts, the Earth can’t reflect as much light, and therefore absorbs more heat. This would result in a change in weather patterns over that area, again, affecting land and marine ecosystems.

9. What is the Great Conveyer Belt?
   It is a system showing the deep cold water currents and the warmer surface water currents which affects climates around the world.

10. How can climate change affect the Great Conveyer Belt?
    Climate change is leading to increased surface ocean temperatures. There is less “perennial” Arctic sea ice (research is showing it to be declining at a rate of 14% per year), more freshwater is being dumped into the oceans – some from increased melting of ice, but more from increased rain and snow due to climate warming- which might decrease the saltiness of the North Atlantic ocean currents, preventing them from sinking and heading on their usual southern route. (Warmer ocean waters from the tropics flow northward to replace the sinking colder water.) Ocean currents determine the local climate on land, and a slowing or stopping of the Great Conveyer Belt will have enormous consequences on countries that depend on the climate for agriculture.

Extensions

• Invite a marine biologist in to talk to the class about climate change and marine ecosystems
• Research ocean salinity and add those numbers to your graph. Use different colours. Examine the trend, and extrapolate the graph, to predict what the concentration might be when you are 50!
• List some ideas that might help change this trend. Is any organization working to do just that?

Credit

Janet Nadeau (nadeau.janet [at] gmail.com)