Aquatic Field Investigation
Materials and Methods 6
Density and Salinity

At this station, your team will measure the density of water, compute the salinity (saltiness) of water, and learn what these measurements tell about the water at your aquatic site. To do this, you will build and use a hydrometer, a device that measures the density (or weight of molecules) in different liquids as compared to water. From these measurements, you'll be able to compute the salinity of the water. Most living creatures can tolerate only a narrow range of salinity. If the salinity in a body of water is too high or too low, many plants and animals will die.


For the Pre-Field station:

For the field station:


Pre-Field Experiment:

First you need a hydrometer. If it has already been built by another group, review steps 1 through 5 and begin at step 6.
  1. Plug one end of the straw with a small piece of modeling clay or plasticine.
  2. Fill the straw one-quarter full with dry sand.
  3. Fill the empty beaker or jar three-quarters full with distilled water (750 ml).

    Put the straw in the beaker, with the modeling clay at the bottom. Be careful not to let water get into the open end of the straw.

  4. Look at how the straw is floating in the water. Is half of the straw above the waterline and half below the waterline? If not, add or subtract sand from the straw until it floats with exactly half of the straw above the water and half below the water.
  5. Using the waterproof marker, make a horizontal line on the straw at the waterline. Label the line "1.00." This is your hydrometer.

  6. Try floating the hydrometer in the jars containing the other liquids (milk, alcohol, and ice water). The chart below indicates the density of each of these liquids. Mark the waterline for each substance and write the appropriate density number. Key this to different colors.

  7. To see how salinity affects water density, empty the beaker from step 3 and refill it three-quarters full (750 ml) with room-temperature water. Slowly add salt to the water, and occasionally stir the solution until the water becomes saturated. You will know when the water has become saturated, because the salt you are adding will begin to accumulate at the bottom of the beaker and no longer dissolve.
  8. Float the hydrometer in the saltwater. Does the 1.0 mark float higher or lower than it did in step 3? Using the waterproof marker, mark the new waterline on the straw.
  9. To see how temperature affects water density, empty the beaker and refill it three-quarters full with hot water. Put your hydrometer in the beaker and note where the waterline is. How does it compare to the waterlines for ice water and room-temperature water? How does it compare to the waterline for milk? What does this tell you about the effect of temperature on water density?
  10. You will take you hydrometer to the aquatic site to help you measure the salinity of water there. First, you will record the density of the water. Then you will use Master 4e, "Density and Salinity Conversion Chart" to convert that reading into a measure of salinity. Practice using Master 4e now by converting the density measurements for distilled water, ice water, alcohol, and milk to salinity measurements.

Field Experiment

To measure the salinity of the water at your aquatic site you will need a 1-liter beaker or jar and your hydrometer.
  1. Fill the beaker or jar three-quarters full (750 ml) with water from the aquatic site.
  2. Place the hydrometer in the water. Which of the marks is the surface of the water closest to? In other words, is the density greater than or less than 1.0?
  3. Using Master 4e, "Density and Salinity Conversion Chart," convert your density reading into a salinity measurement. How does the salinity at your site compare to the salinity measurements you calculated for various substances in the classroom?


After you have conducted both the Pre-Field and Field Experiments, discuss the following questions within your research team and record your answers in your JASON Journal.
  1. Does a hydrometer float higher in dense water than in less dense water? What does higher density indicate about the water?
  2. Describe the relationships among the following three abiotic characteristics: density, salinity, and temperature. (Hint: If one characteristic, such as temperature, increases, what happens to the others?)
  3. What new questions do you have?
  4. Look for more answers to your questions in the JASON Online Systems, the library, and science textbooks.

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Gene Carl Feldman ( (301) 286-9428
Todd Carlo Viola, JASON Foundation for Education (
Revised: 3 Nov 1995