Density - What Difference Does It Make?


Rationale/Objectives/Purpose:
To provide a visual example of the effect of varying density on bouyancy of fluids.
Time Required:
20 minutes
Safety Issues:
Fluids should not be ingested; Possible glass breakage and spills.
Materials:
Procedures:
Teacher:
  1. Mix a selected food color with the corn syrup.
  2. Mix a different color with the water in another container.
  3. Pour the water into the jar.
  4. Carefully pour the oil into the jar.
  5. Carefully pour the corn syrup in the jar.
  6. Start over and do oil first, then corn syrup, then water.
  7. Take two more containers and put hot or warm water in one with a third color.
  8. In the other put cold water with a different coloring.
  9. Pour the warm water slowly into a container already half-full with the cold water.
  10. If done correctly you should be able to see the warm water float on top of the cold water.
Click here to see setup diagram.
Assessment:
Students should be able to tell not only which substance went to which level but also why.
Teacher Background/Extensions:
This can be done either as a teacher demonstration or as a hands-on activity with students working in groups.

 
Background: Density determines the relative bouyancy of objects or fluids. Out of the oil, water, and corn syrup, oil is the least dense so it floats on the water. The corn syrup is more dense so the water floats on top of it. Warm water is less dense than cold water - as the temperature increases, the motion of the tiny water molecules increases and they spread out more, which renders the liquid less dense as a whole. On the other hand, cold water is more dense because the cold water molecules move more slowly and are much closer together.
So how does this all relate to El Niño and/or La Niña events?
Winds blow the surface water. In a La Niña event, stronger trade winds blow more warm surface water away from South America. In an El Niño event, weaker trade winds leave more warm water by South America. The colder (denser) water, which comes partly from melting icebergs, is much deeper. As the deep cold water flows in the oceans, nutrients accumulate in it partly from the sea floor, but mainly from decaying animals and their fecal pellets that sink down through the water column. Upwelling occurs because the winds move the warmer (less dense) surface water by the coast of South America away from the land, leaving a gap, which is filled by water (which is nutrient-rich, cold, dense water) upwelled from beneath. Nutrients stimulate growth of phytoplankton and zooplankton, which in turn provide food for fish, resulting eventually in greater fish production. During an El Niño, upwelling is reduced and there is more warm water (which again, is less dense). (Recall:  The depth at which the cold water and warm water meet in the oceans is called the thermocline. The temperature changes rapidly as one goes through the thermocline).