Physical Oceanography Lesson Plan
Objective | Materials | Teacher Background | Activity | Questions | Answers
Teachers: This activity will help students to understand how salinity affects the density of water. In addition, students will better understand how pure and salt water systems work together. With a little extra guidance, they will be able to apply this knowledge to the circulation of the open ocean and to such practical matters as sewage and waste water disposal.
Objective
To observe how different water densities control the depth at which
different water masses occur.
Three sturdy plastic drinking cups
Spoon for stirring
Two transfer pipettes/ eye dropper type
Five types of water:
1. clear tap water (0 ppt)
2. tap water dyed with blue food coloring (0 ppt)
3. clear very salty water(400 ppt = 40 g per liter; one liter per group)
4. slightly salty water dyed with red food coloring (100 ppt = 10g per liter, one liter per group
5. very salty water dyed with green food coloring (400 ppt, one liter per group)
Temperature and salinity (the amount of dissolved salts in the water) affect the density of the water. Salinity of different ocean waters varies by very small amounts, so oceanographers need to be accurate when measuring salinity. Changes in density caused by wind and currents at the surface affect the deep-ocean currents. Density ultimately effects the objects in the water, such as organisms (everything from whales to bacteria, plankton and seaweed), and ships and submarines. The saltier the water, the more buoyant an object becomes. Therefore, salt water is more dense, and will sink more, thus very salty is found at the bottom. Less salty water is less dense and will float on top of the more dense salty water.
Of course, the layers are more complicated
than this, but for this activity, students should be able to understand that
salt or fresh water drops are going to want to "hang out" in water with
similar properties. So fresh water drops will rise to the fresh water layer
and salt water drops will sink to the salt water layer. The salinity of the
water mixes, or changes, only when vigorously stirred. Though students
learn early on that the ocean contains salt water, it does not mean that all
the water has the same amount of dissolved salt. Help students understand
how density affects circulation of ocean waters and ramifications for
practical applications such as sewage discharge. 
Activity
The five types of water have been prepared and are in labeled
containers; make sure you get one of each type at your lab station.
Part I
1. Label one cup "tap water" and fill it half full with clear tap water
2. Fill one medicine dropper with very salty green water
3. Place one drop of very salty green water into the cup with clear water
4. Record observationsPart II
1. Label one cup "salty water" and fill it half full with clear salt water
2. Fill the other medicine dropper with blue tap water
3. Place one drop of blue tap water into the cup with clear salt water
4. Record observationsPart III
1. Label one cup "salty water" and fill it half full with very salty green water.
2. Place a piece of plastic wrap in contact with the surface of the water. Pour about a quarter cup full of clear tap water slowly onto the plastic wrap and discard it.
3. Record observations.
4. Making sure that the dropper is clean, fill the dropper with slightly salty red water.
5. Place the dropper into the layer of very salty green water and squeeze out a drop of slightly salty red water.
6. Record observations.
7. Take the same dropper of slightly salty red water and place it into the layer of clear tap water and squeeze out a drop of slightly red water.
8. Record observations.
9. Using the spoon, mix the layered water system together.
10. Record observations.
1. What happened to the drop of very salty green water in the tap water? Why?
2. What happened to the drop of blue tap water in the salty water? Why?
3. What did the tap and the very salty green water not mix together?
4. What is this two-layer system called? Explain this system.
5. What happened when the two drops of slightly salty red water were added to different layers? Why?
6. What is the name of the system when the drops of different salinities were added to the layered system? Explain this system.
7. What is the name of the system in Part III step 9? Explain this system.
8. How do oceanographers measure the different densities out in the open ocean?
9. Why is this information important? List as many reasons as you can.
10. Who would this information be important to?
1. Sank because salt water is "heavier" due to extra dissolved salts causing it to be more dense than the surrounding water; similar to why a rock with sink in a puddle.
2. Floated to the top because fresh water is "lighter" or less dense than the surrounding water; similar to why a piece of wood will float on top of water (until it is waterlogged).
3. Different densities do not mix unless they are stirred or heated (to increase the rate of diffusion of the dissolved salts).
4. Stable density distribution: more dense salt water is at the bottom and less dense fresh water is on top.
5. Moved to balance density area within the two-layered system.
6. Unstable density system; the densities were not stable and the drop will move to the best place in between two layers that best fits its density.
7. Neutral stable system: the different waters mixed together, there are no more separate densities, therefore the waters are "neutral" in densities; nowhere in the water is one area more or less dense than the other.
8. By measuring salinity and temperature and computing density.
9. Difference in density will cause water to move or may prevent its movement. For example, in summer, the sun warms the surface water and it becomes less dense, thus less likely to sink and stability of the water column is increased. In the polar regions, ice formations causes increased salinity in the remaining liquid water and it becomes more dense and sinks. This sets up major ocean circulation patters. In the Mediterranean Sea, warm surface water evaporates and produces salty water that sinks into the North Atlantic, another major ocean circulation pattern. The fate of sewage discharges (generally warm, low salinity water) into ocean will be determined by the relationship between the density of the discharge and that of the receiving waters. Submarines are submerged in the water in salinity and density would cause the submarine to rise or sink.
10. Sailors concerned with circulation patterns of the ocean, climate forecasters concerned with the transport of heat by the ocean; navies of the world since they are responsible for naval fleets throughout the world's oceans.