Exploring Our Fluid Earth

Water movement caused by differences in both temperature and salinity is called thermohaline circulation (thermo come from the Greek word for temperature, haline comes from the Greek word for salt). This activity models thermohaline circulation.

• Two transparent plastic cups (thin enough to be easily punctured)
• Nail or pencil
• Four pieces of masking tape
• Transparent plastic or glass box (~shoebox sized)
• Fresh water
• Salt water
• Salt 
• Two sheets of white paper
• Two beakers
• Food coloring
• Heat source
• Thermometer
• Tongs or hot pads
• Ice bath (ice cubes and water in container)
• Pennies (~30) or other weights
• Balance 
• Ruler
• Colored pencils or crayons (same colors as food coloring)

1. Study the apparatus shown in Fig. 2.19. Make colored sketches to show what you think the system will look like after 1 minute and after 10 minutes once the system is set in motion.
2. Set up a model thermohaline system:
   
   1. Obtain two transparent plastic cups. Make two holes in each cup as shown in Fig. 2.19. You can make the holes either by firmly twisting a pencil point or nail into the plastic or by melting holes in the plastic with a hot nail.
   2. Cover both holes on each cup with one piece of masking tape. Make a tab or keep one end of the tape loose so that the tape can be removed easily. If your plastic cups are angled (as in Fig. 2.19), the masking tape should be placed on the inside of the cups so it can be easily removed.
   3. Put the cups next to the plastic shoebox. Fill the shoebox with clear, colorless room-temperature water to a level above the two holes on each cup but at lower than the rim of the cups.
   4. Put white paper behind and under the plastic shoebox.
   5. Put red-colored fresh water into a beaker and heat it to 50˚–70˚C. 
   6. Put blue-colored salt water into a beaker. Put it in an ice bath to cool it to about 5˚C. 
   7. Put the plastic cups in the water in the shoebox. Have your lab partner hold the cups down. If the cups are unstable, add pennies or other weights. If desired, preheat the pennies for the hot water cup and chill the ones for the cold-water cup.
   8. Fill one of the plastic cups with the heated red water up to the shoebox waterline. Fill the other plastic cup with the chilled blue water up to the shoebox waterline. 
3. Make sure the holes in the two cups are facing each other. Peel off all of the masking tape, as quickly and as smoothly as you can, disturbing the water as little as possible. 
4. Observe the movement of the water from the side of the shoebox and from the top. Make a series of colored sketches to record the pattern of any flow of water:
   
   1. as soon as the tapes are removed.
   2. after 1 min.
   3. after 5 min.
   4. after 10 min.
5. Repeat Procedures 1–4 testing your own combination of temperature and/or salinity.