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Activity: Gravitational Currents

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

Materials for Parts A–C

  • Two wide-mouth bottles
  • Food coloring
  • Four large cups
  • Cards with a plastic coating (e.g., playing cards, identification cards)
  • Fresh water
  • Tray
  • Two beakers
  • Fig. 2.18
  • Table 2.5
  • Towels
  • Colored pencils (optional)

Additional Materials for Parts A and C

  • Salt water

Additional Materials for Parts B and C

  • Two beakers
  • Heat source
  • Ice bath (container in a tray of ice cubes and water)
  • Thermometer
 

 

Procedure
Safety Note: When preparing hot water and handling hot containers, use tongs or heat-resistant gloves or gripping devices.

 

A. Test the effect of salinity on gravity currents.

  1. Predict the movement of salt water and fresh water at room temperature when
    1. fresh water is in the top bottle and salt water is in the bottom bottle
    2. salt water is in the top bottle and freshwater is in the bottom bottle.

Record your predictions in Table 2.5.The water in the bottles may

  1. not move (the fresh and salt water do not mix), 
  2. switch places (the water in the bottom bottle switches places with the water in the top bottle), or
  3. mix together (the fresh and salt water mix together and become brackish water).

 

  1. Observe as your teacher demonstrates the procedure for setting up a two-liquid system. Do these steps over a tray or towel.
    1. Put a few drops of food coloring into one of the bottles.
    2. (Optional) Put a few drops of a different food coloring into the second bottle.
    3. Overfill one bottle with one liquid and overfill the second bottle with the second liquid. Overfill the bottles so that no air bubbles are trapped inside (Fig. 2.18 A).
    4. Place a plastic card over one of the bottles. Holding the card snugly to the bottle, turn the bottle over (Fig. 2.18 B).
    5. Place the upside-down bottle on top of the other bottle (Fig. 2.18 C).
    6. Carefully remove the card from between the bottles (Fig. 2.18 D).

<p><strong>Fig. 2.18.</strong> Steps in manipulating bottles of liquids to observe gravitational flow between two liquids. (<strong>A</strong>) Add food coloring to one of the bottles and pour liquid into each bottle. (<strong>B</strong>) Invert the top bottle using a card to hold the liquid in the bottle. (<strong>C</strong>) Place the two bottles together. (<strong>D</strong>) Carefully remove the card.</p><br />

 

  1. Test each of the two-liquid salinity systems described in procedure 1 using the steps in procedure 2. You will be able to observe the movement of the fresh and salt water by observing the color of the water.
    1. Observe the system at 10 seconds, 1 minute, and 5 minutes.
    2. The two water masses may
      1. not move (the colors do not mix),
      2. switch places (the colors switch places), or
      3. mix together (the color mix together)
         
  2. Compare your results with those of your classmates.

 

B. Test the effect of temperature on gravity currents.

  1. Predict the movement of hot and cold fresh water when
    1. hot water is in the top bottle and cold water is in the bottom bottle, and
    2. cold water is in the top bottle and hot water is in the bottom bottle.

Record your predictions in Table 2.5.

 

  1. Prepare hot water and cold water.
    1. To prepare hot water, place a beaker of water on a hot plate and heat to 50˚C to 70˚C
    2. To prepare cold water, place a beaker of water in an ice bath until the water in the beaker reaches a temperature of ≈5˚C. Do not put ice cubes in the beaker.
       
  2. Test each of the two-liquid temperature systems using the steps described in procedure 2.
    1. Record your observations in Table 2.5.
    2. Use colored sketches if desired to record movement of the liquids.
       
  3. Compare your results with those of your classmates.

 

C. Test the effects of both salinity and temperature on gravity currents.

  1. Design two experiments to test the effects of both temperature and salinity on the movement of water in bottles. For example, you might want to determine what happens when cold salty water is in the top bottle and room temperature fresh water is in the bottom bottle.
     
  2. Record your two-liquid systems and predict their movement in Table 2.5.
     
  3. Test your experimental systems using the steps described in procedure 2.
    1. Record your observations in Table 2.5.
    2. Use colored sketches if desired to record movement of the liquids.
       
  4. Compare your results with those of your classmates.

 

Activity Questions: 
  1. Write a description of the interaction between the gravitational force and the buoyant force when
    1. fresh water is in the top bottle and salt water is in the bottom bottle (example 1 in Table 2.5)
    2. salt water is in the top bottle and fresh water is in the bottom bottle (example 2 in Table 2.5).
       
  2. Explain sinking, rising, and floating in two-liquid systems in terms of
    1. the densities of the two liquids
    2. the forces of gravity and buoyancy.
       
  3. Examine your data in Table 2.5. How well did your observations agree with your predictions? How might any differences be explained?
     
  4. For each of the two-liquid systems in Table 2.5, name a geographic location and scenario where such conditions might occur. For example, if the top layer was room temperature fresh water and the bottom layer was cold salt water, a possible location and scenario would be when rain falls into cold seawater in a bay in Maine.
     
  5. How do changes in density influence the formation of currents?
     
  6. Arrange the following list from most dense to least dense:
    1. seawater at the surface in a tropical rainstorm.
    2. seawater at the surface around Antarctica during winter.
    3. seawater at the surface in tropical sun.
    4. seawater at the surface in the tropics at night.
    5. seawater directly beneath a mass of polar ice.

Table of Contents:

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.