To compare salt water with freshwater, you need four small, clear bottles, such as water bottles, with equal-sized openings; masking tape; a fine-tip permanent marker; an index card; table salt; two containers that have a greater capacity than the bottles; a measuring cup; a triple-beam balance scale; a stirring spoon; two different food colorings; a cookie tray or shallow pan; and a clock with a second hand.
Record your progress as you prepare for and conduct the experiment. Label two bottles and one container "+ Salt." Label the remaining two bottles and container "Fresh." Add the same number of cups of water to each container. On the triple balance beam, weigh 8 grams of salt per cup of water used in the container. Add the salt to the container marked "+ Salt," and stir until the salt completely dissolves. The salinity of the salt water in the container is about the same as that found in the deep ocean.
Place the cookie sheet or shallow pan on the table and work over it to catch spills. Add three drops of food coloring to the salt water container and three drops of a different food coloring to the freshwater container. Mix the water in each with a clean utensil. Fill a "+ Salt" bottle and a "Fresh" bottle. Add the remaining salt water and freshwater to the appropriate remaining bottles to serve as color samples. Set these bottles aside.
Place the index card over the mouth of the "+ Salt" bottle. Hold the bottle at its base with one hand and the index card with two fingers of your other hand. Keep the index card tightly pressed against the bottle's opening and carefully invert the bottle on top of the opening of the "Fresh" bottle. Check that the openings of the bottles are lined up with the index card still in place. Record the time, and carefully pull the index card from between the two bottles.
Observe what happens. Look for color changes or wavy lines called schleiren. These lines sometimes appear as the water from the two bottles mix because the salt water and freshwater have different densities and different refraction indices. They are similar to the wavy lines seen over hot pavement caused by the less dense rising hot air mixing with the more dense cooler air. Compare the original colors of the bottles to what you see. The food coloring moves with the water into which it was put. Record anything else of interest.
Repeat the experiment two more times with the "+ Salt" bottle on top and three times with the "Fresh" bottle on top. Prepare your data in a table or graph. Based on this data that compares salt water vs. freshwater, which is more dense? Saltwater has a greater density than freshwater because the salt fills up the small spaces between water molecules. The colored salt water sinks because of its higher density. You float on freshwater because your density is slightly less than that of the water. You float more easily on salt water because of the greater difference between your density and that of salt water.