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Question Set: Using a Hydrometer to Determine Density and Salinity
NGSS Science and Engineering Practices
NGSS Crosscutting Concepts

Fig. 2.13. Lines define relationships between temperature (red vertical lines), density (grey horizontal lines), and salinity (blue curved lines).

Image by Byron Inouye

Image caption

Fig. 2.14. A hydrometer used to determine water densities in g/mL. The pink shaded region indicates the optimum density of saltwater aquaria at an average temperature of 20˚C to 25˚C.

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Image by Byron Inouye


  1. On the hydrometer in Fig. 2.14, determine the density range in g/mL for the following types of water shown below with their respective salinity concentrations in ppt:
    1. Normal seawater (approximately 35 ppt)
    2. Fresh water (less than 1 ppt)
    3. Brackish water (between 33 and 1 ppt)
    4. Hypersaline water (greater than 38 ppt).
  2. Why do seawater, brackish water, hypersaline water, and fresh water each have a range of densities?
  3. Study Fig. 2.13, then answer the following questions:
    1. As salinity increases, what happens to density?
    2. If density increases, what factors might be changing?
    3. At the salinity of average seawater, what happens to density as the temperature increases? Is this also true of fresh water? Brackish water?
  4. The pink shaded section on the hydrometer shown in Fig. 2.14 indicates the optimum density of saltwater aquaria at an average temperature of 20˚C to 25˚C. How does this compare to the density range of open-ocean seawater?
  5. If a hydrometer in an aquarium shows a density reading of 1.025 g/mL with the water temperature at 20˚C,
    1. what is the salinity of the water?
    2. would the water be described as brackish water, average seawater, or hypersaline water?
  6. Assume that you have a saltwater aquarium maintained at room temperature (25˚C). A hydrometer reading shows that its density is 1.040 g/mL. To bring the aquarium back within the normal seawater range, select one course of action from each of the questions below. Explain your selection.
    1. Would you add fresh water or salt water?
    2. Would you remove the excess salts from the sides of the aquarium or wash the evaporated salts back into the aquarium?
  7. The left vertical scale in Fig. 2.13 gives density in g/mL. Label the scale to show fresh water, brackish water, seawater, and hypersaline water.
  8. On Fig. 2.13 lightly shade in the average seawater salinity range between 33 ppt and 38 ppt for the entire width of the graph. Use your shading to determine the density range of seawater.
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.