Can Water Float on Water?


Objective

The goal of this project is to investigate what happens to layers of water with different densities. You will investigate density differences caused by both temperature and salinity.

Introduction

Water covers 70% of Earth's surface. Seen from space, the blue of the oceans and the white of clouds are the dominant visual features. The water of the oceans is not uniform. Climatic processes create large-scale differences in ocean water temperature and salinity, illustrated in the first two maps, below. As you might expect, ocean waters near the equator tend to be warmer than those at higher latitudes. The first map shows sea surface temperature, coded in color (see legend).

Map of sea surface temperature.
Color-coded map of sea surface temperature.

The second map shows global differences in ocean surface salinity (dissolved salt concentration). At the surface, in general salinity is higher in equatorial regions and lower at the high latitudes.

Map of sea surface salinity.
Color-coded map of sea surface salinity in parts per thousand (ppt or 0/00).

What goes on at the ocean surface does not tell the whole story. The ocean has depth, too. In the deep ocean, huge masses of water circulate around the globe, driven by differences in temperature and salinity. This is called the thermohaline circulation, sometimes also known as the global conveyor belt. Differences in temperature and salinity cause differences in ocean water density. As water warms, it expands, decreasing density. As salt concentration rises, density increases, because the salt molecules can occupy spaces between the water molecules. Denser water sinks beneath water that is less dense. As denser water sinks, water must rise somewhere to replace it. As you are doing your background research for this project, you should read up on how the thermohaline circulation works.

In this project, you will do experiments to see what happens when layers of water at different densities are brought together. You'll create your two "layers" in plastic or glass bottles, coloring them with different food colors to tell them apart. Then, you'll bring the two layers together by flipping one bottle over on top of the other (we'll tell you how to do it without spilling half the bottle!). You can see for yourself if water can float on water.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

  • salinity,
  • density,
  • hydrometer,
  • thermohaline circulation,
  • estuary.

Questions

  • How does the density of water change as a function of temperature?
  • How does the density of water change as a function of dissolved salt?

Bibliography

Materials and Equipment

To do this experiment you will need the following materials and equipment:

  • 4 (or more) small, clear bottles with equal-sized openings (plastic water bottles work well),
  • masking tape,
  • permanent marker,
  • business card or index card, (or stiff plastic of similar thickness),
  • table salt,
  • 2 containers for mixing and pouring solutions (should have larger capacity than bottles),
  • measuring spoon,
  • stirring spoon,
  • 2 different food coloring colors (e.g., red and blue, blue and yellow, red and yellow),
  • tray or shallow pan to work over (for spills),
  • towels for clean-up,
  • hot and cold tap water,
  • ice,
  • clock.

The following materials and equipment are optional:

  • hydrometer (available at stores that sell marine aquariums),
  • thermometer (range at least 0–50°C).

Experimental Procedure

Salinity and Mixing

  1. You'll need to keep track of which containers have salt added and which ones do not, so start by labeling your containers while everything is still dry. Mark one container and two bottles "+ salt." Mark the other container and two bottles "fresh."
  2. Add tap water to each container.
  3. Add salt to the "+ salt" container and stir until it dissolves. How much salt? You decide! Use the following information to assist you with your calculations:
    1. The salinity map in the Introduction shows that deep ocean salinity ranges from 32 to 37.5 parts per thousand (ppt or 0/00). As an example, 32 ppt would mean 32 g of salt per 1000 g of seawater.
    2. At 25°C, the maximal solubility of NaCl in water is about 357 g/l.
    3. For making measurements in a typical American kitchen, the following facts will be helpful. A cup of table salt is approximately 292 g (GourmetSleuth.com, 2001). There are 16 tablespoons per cup, and 3 teaspoons per tablespoon. Finally, a cup of water is the same as 236.6 ml.
  4. Optional: if you have a hydrometer, measure the density of each solution. Rinse off the hydrometer and wipe the outside dry between measurements so that you don't transfer one solution to the other.
  5. Add about 3 drops of food coloring to each container. Use one color for "+ salt" and a different color for "fresh." Note which is which in your lab notebook. (You'll want your notebook handy, but off to the side in case of spills.)
  6. Completely fill a "+ salt" bottle with colored salt water.
  7. Completely fill a "fresh" bottle with colored fresh water.
  8. Use the two remaining bottles for color samples of each solution for comparison as the experiment proceeds.
  9. Now comes the tricky part. You are going to invert one bottle and put it on top of the other, without spilling. It doesn't matter which one you choose to flip over first, because you'll be doing the experiment both ways. It's a good idea to practice this maneuver first with plain tap water until you get the hang of it, so you don't waste your solutions. Here's how:
    1. Use the card (or plastic) to cover the top of the bottle you're going to invert.
    2. Hold the bottle near the base with one hand while holding the card against the opening with two fingers of the other hand.
    3. Slowly and carefully flip the bottle over, keeping the card pressed tightly against the opening. For plastic bottles, try not to squeeze the bottle as you do this, since squeezing will push water out of the bottle. Holding near the bottom of the bottle where it is stiffer will help.
    4. Place the inverted bottle on top of the other bottle (the card remains in place, so it is between the openings of the two bottles).
    5. Line up the two bottles so that the inverted bottle is balanced on top.
    6. Note the time, and then carefully slide the card out from between the two bottles.
    7. With practice, you'll be able to do this without spilling more than a few drops.
  10. Observe what happens to the two solutions. Write your observations in your lab notebook. Remember to the note the time as you make your observations. Here are some things to look for:
    1. Do you see any evidence of mixing (e.g., color changes, or schlieren lines)? Note: schlieren lines are wavy lines caused by changes in the index of refraction of the solution. Since the two solutions have different densities, they will also have different indices of refraction. Where the two solutions mix, schlieren lines may be apparent. You may have seen schlieren lines before on a hot summer day in the air over hot asphalt pavement. In this case the lines are the result of rising hot air mixing with cooler air above.
    2. How does the color of solution in each bottle compare to the original color?
    3. Is the color uniform throughout each bottle?
    4. Note anything else of interest.
  11. Optional: if you have a hydrometer, measure the density of the water in each bottle at the conclusion of the experiment.
  12. Confirm your results by repeating the experiment. You should perform at least three trials with salt water in the top bottle and fresh water in the bottom bottle, and at least three trials with fresh water in the top bottle and salt water in the bottom bottle.

Temperature and Mixing

  1. In the second experiment you'll investigate the effect of water temperature on mixing. This time, you'll use fresh water in both bottles. Label your containers "hot" and "cold."
  2. Add hot tap water to one container, and cold tap water to the other. (Note: since you are bound to spill some water, make sure that the "hot" water is not so hot that it would scald.)
  3. Optional: if you have a thermometer, measure the temperature of each solution. Rinse off the thermometer and wipe the outside dry between measurements so that you don't transfer one solution to the other. You can also measure the density of each solution with a hydrometer, if you have one.
  4. Add about 3 drops of food coloring to each container. Use one color for "hot" and a different color for "cold." Note which is which in your lab notebook. (You'll want your notebook handy, but off to the side in case of spills.)
  5. Completely fill a "hot" bottle with colored hot water.
  6. Completely fill a "cold" bottle with colored cold water.
  7. Use the two remaining bottles for color samples of each solution for comparison as the experiment proceeds.
  8. Follow the instructions above (step 8) for inverting one bottle over the other.
  9. As before (step 9, above), observe what happens to the two solutions. Write your observations in your lab notebook. Remember to the note the time as you make your observations.
  10. Optional: if you have a thermometer, measure the temperature of the water in each bottle at the conclusion of the experiment. Measure the density of the solution in each bottle if you have a hydrometer.
  11. Confirm your results by repeating the experiment. You should perform at least three trials with hot water in the top bottle and cold water in the bottom bottle, and at least three trials with cold water in the top bottle and hot water in the bottom bottle.

For your presentation, think about how your results relate to mixing of ocean water when currents carrying water at different temperatures or salinities meet. Alternatively, you might want to try relating your results to estuaries, where fresh water flowing from streams and rivers meets the ocean and its tides.

Variations

  • Try different colors (e.g., lighter color for denser fluid and vice versa). You may notice fluid movements that you missed previously.
  • Try varying the salt concentration. For example, if you cut the amount of added salt in half, is mixing time affected? What do you think will happen?
  • Try intermediate temperatures. What do you think will happen to mixing time?
  • What do you think would happen if you tried warm salt water over cold fresh water?
  • Try different temperatures of salt water. To make sure that the salt concentration is equal, start with a single salt water solution (make enough to more than fill two bottles). Split the solution in half. Add dye to each half. Chill one of the solutions in a tightly-covered container in the refrigerator or freezer. Warm the other solution on the stove using very low heat. The solution should not become too hot too touch. Keep it covered so you don't lose water vapor, which would increase the salt concentration in the remaining solution.
  • For a different way of looking at the density of salt water, check out the Science Buddies project: Try This Eggsperiment to Measure the Density of Salt Water.

    Credits

    Andrew Olson, Ph.D., Science Buddies

    Sources


    Last edit date: 2007-06-27 10:20:27

 

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