Seawater composition: an introduction
Skills and concepts that students must have mastered
How the activity is situated in the course
Content/concepts goals for this activity
Through the activity, students will review the hydrologic cycle and learn where the cycle transports solutes.
Higher order thinking skills goals for this activity
- Applying existing knowledge (building the hydrologic cycle from memory)
- Extending the conceptual framework (hydrologic cycle) to new processes (transport of solutes)
Other skills goals for this activity
Description and Teaching Materials
The activity is a two part worksheet. The first part asks students to list Earth's major reservoirs of water, rank the reservoirs by size, suggest transport processes between reservoirs, and think about transfer rates between reservoirs. This part takes about 10 minutes and is done individually. The instructor then calls on students to supply one answer at a time; this is most effective if the answers are recorded pictorially at the board by the instructor or by students. Then students similarly rank reservoir size and transfer rates, with the instructor supplying corrections and background information as needed. An important part of the activity is ensuring that students revise their worksheets based on the classwork.
In the second part, students determine which portions of the hydrologic cycle are likely to transport solutes, and begin to think about the types and relative amounts of substances that comprise seawater. Lastly students predict if the same substances will be found in fresh water, and if they are more concentrated there. This part of the activity also takes about 10 minutes.
Together, the activity and subsequent discussions require an entire 50 minute class period.
The activity helps introduce a unit on seawater chemistry.
Teaching Notes and Tips
I find that conceptually connecting the transfer rates of water molecules in the hydrologic cycle, which students learn in the materials above, to the residence time of solutes in seawater is a helpful extension of this activity. For example, students know that the cycling of water in the atmosphere is rapid based on their experience and the classwork. This atmospheric cycle can be formalized as a "residence time": (Size of reservoir)/(rate of input or output) = residence time. For the atmosphere, this is (13,000 km3)/(434,000 km3/year) = 0.030 years = 11 days; in this case the rate of input is the global rate of evaporation, which also equals the global rate of precipitation. This is basically the same calculation one uses to estimate residence times of solutes in seawater: (total mass of solute in ocean)/(rate of input or output of solute).