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
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.
- Predict the movement of salt water and fresh water at room temperature when
- fresh water is in the top bottle and salt water is in the bottom bottle
- 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
- not move (the fresh and salt water do not mix),
- switch places (the water in the bottom bottle switches places with the water in the top bottle), or
- mix together (the fresh and salt water mix together and become brackish water).
- Observe as your teacher demonstrates the procedure for setting up a two-liquid system. Do these steps over a tray or towel.
- Put a few drops of food coloring into one of the bottles.
- (Optional) Put a few drops of a different food coloring into the second bottle.
- 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).
- Place a plastic card over one of the bottles. Holding the card snugly to the bottle, turn the bottle over (Fig. 2.18 B).
- Place the upside-down bottle on top of the other bottle (Fig. 2.18 C).
- Carefully remove the card from between the bottles (Fig. 2.18 D).

- 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.
- Observe the system at 10 seconds, 1 minute, and 5 minutes.
- The two water masses may
- not move (the colors do not mix),
- switch places (the colors switch places), or
- mix together (the color mix together)
- Compare your results with those of your classmates.
B. Test the effect of temperature on gravity currents.
- Predict the movement of hot and cold fresh water when
- hot water is in the top bottle and cold water is in the bottom bottle, and
- cold water is in the top bottle and hot water is in the bottom bottle.
Record your predictions in Table 2.5.
- Prepare hot water and cold water.
- To prepare hot water, place a beaker of water on a hot plate and heat to 50˚C to 70˚C
- 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.
- Test each of the two-liquid temperature systems using the steps described in procedure 2.
- Record your observations in Table 2.5.
- Use colored sketches if desired to record movement of the liquids.
- Compare your results with those of your classmates.
C. Test the effects of both salinity and temperature on gravity currents.
- 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.
- Record your two-liquid systems and predict their movement in Table 2.5.
- Test your experimental systems using the steps described in procedure 2.
- Record your observations in Table 2.5.
- Use colored sketches if desired to record movement of the liquids.
- Compare your results with those of your classmates.