If you have the advantage of visiting the ocean or the Great Salt Lake in Utah this summer, you may find your swimming experience in these bodies of water to be slightly different than swimming in a freshwater lake or river. If you accidentally get water in your mouth or eyes from the ocean or the Great Salt Lake, you will certainly notice the saltiness of the water. But what about floating? Is it easier to float in the ocean or Great Salt Lake than in freshwater? And if there is a difference in your ability to float, do you think that water from one body of water is denser than water from another body of water? Which do you think is the most dense? Freshwater from a lake, saltwater from the ocean, or saltwater from the Great Salt Lake? Do this experiment to find out!

What You Need:

  • 3 test tubes (or use 3 jars instead)
  • Jar, drinking glass, or beaker (for making the salt solutions)
  • Water
  • Salt (you can use just regular table salt)
  • Medicine dropper
  • Red, blue, and yellow food coloring
  • Graduated cylinder
  • Measuring cup or beaker that measures in 10’s of ml
  • Teaspoon


What You Do:

  • Add one drop of blue food coloring to the test tube of freshwater, one drop of yellow food coloring to the test tube of “ocean water”, and one drop of red food coloring to the test tube of “Great Salt Lake water.” As each test tube of water receives a drop of food coloring, closely observe how the food coloring mixes with each type of water. Can you draw any conclusions or predictions about which type of water will be the “heaviest” based on what you observed with the mixing of the food coloring? After making your predictions, stir up each solution so that the color is uniform throughout each test tube.
  • Fill the dropper with water from the “Great Salt Lake” test tube, and add it to the graduated cylinder. You may need to add two or three droppers full of water to the cylinder.
  • Rinse the dropper out really well with faucet water to avoid cross contamination of salt and food coloring.
  • Fill the dropper with water from the “ocean” test tube. This time, very gently and carefully add it to the graduated cylinder so that the force of the water being squeezed out of the dropper doesn’t mix the two waters. Add about the same amount of “ocean water” to the graduated cylinder as there is “Great Salt Lake water.” Rinse out the dropper with faucet water.
  • Fill the dropper with water from the “freshwater” test tube. Again, add the water very gently to the water already in the graduated cylinder to avoid mixing the water, and add about the same amount of freshwater as “ocean water.”

What Happened:

Density is the measure of how much matter (mass) is packed into an item or material compared to the amount of space (volume) it takes up. A material that is more dense (e.g. lead) will weigh more than a material that is less dense (e.g. cork) even though they both take up the same amount of space. Or, to think of density another way, 10 pounds of cork takes up a lot more space than 10 pounds of lead.

In this experiment, the salt added to the “ocean water” and “Great Salt Lake water” caused these solutions to contain more matter than what was in the freshwater even though the different types of water still took up the same amount of space. The more salt in a solution, the more dense or “heavier” it is, and the less salt in a solution, the the less dense or “lighter” it is. This allows the “ocean water” to float on top of the “Great Salt Lake water” and the freshwater to float on top of the “ocean water.”

To really prove that the “Great Salt Lake water” is the most dense, the freshwater is the least dense, and the “ocean water” has a density somewhere in between these two types of water, try this experiment again except this time reverse the order that the solutions were placed in the graduated cylinder. Do they sit on top of each other as they did before or do they mix up?