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Activity: Measuring Salinity

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

Materials

  • Skewer
  • Centimeter ruler
  • Scissors
  • Pen
  • Paperclip
  • Small shell vial, ≈3mL volume
  • Cork to fit vial
  • Fast-drying varnish
  • Sand
  • Solutions of known salinities
  • Thermometer
  • Towels
  • Figs. 2.15, 2.16, and 2.17
  • Table 2.4
  • Quick-drying glue for securing hydrometer (optional)
     

<p><strong>Fig. 2.15.</strong> Measuring and marking a skewer to construct a hydrometer</p> <p><strong>Fig. 2.16.</strong> Make the scale-and-cork assembly by (<strong>A</strong>) marking the center of the cork (<strong>B</strong>) starting a hole in the cork with a paper clip (<strong>C</strong>) inserting the skewer into the cork and (<strong>D</strong>) darkening the mark at which the skewer meets the cork.</p><br />

 


Procedure

  1. Mark the hydrometer scale on a wooden skewer. See Fig. 2.15.
    1. Cut a skewer to 7.5 centimeters (cm) long. Cut off the blunt end and leave the sharp point on your 7.5 cm piece.
       
    2. Starting at the unsharpened end, make dark lines completely around the skewer every 1 cm with a pen. Mark every 0.5 cm with a dot.

       

  2. Make the scale-and-cork assembly following the steps in Fig. 2.16.
    1. Mark the centers of both ends of the cork (Fig. 2.16 A). Write your initials on the side of the cork so that you will be able to identify your hydrometer.
       
    2. Use a partially straightened paper clip to start the hole in the cork in which you will insert the skewer (Fig. 2.16 B).
       
    3. Put glue on the sharpened end of the skewer and insert it into the cork until the tip of the skewer sticks out of the other end of the cork (Fig. 2.16 C).
       
    4. On the skewer, darken the mark at the top of the cork with a pen so that if the skewer is pulled out, it can be reinserted to the same depth (Fig. 2.16 D).

       

  3. Add weight to the vial to finish making the hydrometer.
    1. Put sand in a small shell vial.
      1. The amount of sand you will need will vary with the type of sand and size of the vial.
      2. In a 3 mL vial the depth of sand should be about 1.5 cm.
      3. Gently tap the shell vial on the counter to pack the sand firmly.
      4. Insert the scale-and-cork assembly into the vial. Push down on the cork.
         
    2. Test the hydrometer in fresh water and in very saline water. Adjust it by adding or removing dry sand. When you are finished, the hydrometer should float as follows:
      1. When placed in fresh water, it will not sink completely. About 1 cm of the scale will remain above the water.
      2. When placed in very saline water, most of the skewer will be above water, but the cork will remain just under the surface.
         
    3. Once the hydrometer is adjusted, dry the cork carefully without changing its position. Seal the cork into the vial by lightly covering it with clear spray varnish. It is important that the varnish has time to dry before continuing with procedure 4.
       
  4. Standardize your hydrometer.
    1. Record the salinity of the solutions provided by your teacher in Table 2.4.
       
    2. All of the solutions should be at the same temperature.
      1. Verify that all the solutions have a similar temperature by measuring the temperature of the solutions in degrees Celsius.
      2. Record the temperature in Table 2.4.
         
    3. Place your hydrometer in the distilled water sample.
      1. Twirl it gently to overcome surface tension.
      2. Determine how much of the skewer is above the surface of the water. If possible, estimate the measurement in millimeters (mm). Remember to read your scale starting from the unsharpened end of the skewer.
      3. Record the measurement in Table 2.4.
      4. Dry the hydrometer.
         
    4. Repeat procedures 4b and 4c with the remaining standard solutions. Work from least to most saline, drying the hydrometer after each test to avoid contaminating the solutions.

       

  5. Use Fig. 2.13 to determine the density of each solution at the measured temperature. Record the density in Table 2.4. For example, if the solution has a salinity of 10 ppt at 25˚C,
    1. Locate 25˚C on the horizontal axis of the graph in Fig. 2.13.
       
    2. Move vertically along this line until it intersects the 10 ppt salinity curve.
       
    3. Move left horizontally to the density scale, 1.004 g/mL.

       

  6. Make a graph in Fig. 2.17 of the standardization data recorded in Table 2.4.
    1. Record the temperatures of the test solutions in the space provided.
       
    2. For each solution tested, mark the data point on the graph where the density and hydrometer readings meet.
       
    3. Draw the line that best fits the plotted data points.

       

  7. Obtain one or more unknown solutions from your teacher.
    1. Use your hydrometer to determine the density of each liquid.
    2. Record your procedure and your results.

 

Activity Questions: 
  1. What are the highest and lowest readings, estimated in millimeters, that you can make with your hydrometer? How could you determine the salinity and density of each?
     
  2. Explain why the small diameter of the skewer increases the sensitivity of the hydrometer. Why not use a thicker skewer?
     
  3. Explain how differences in the densities of the liquids affect the rising and sinking of the hydrometer. How does a change in salinity affect the hydrometer reading?
     
  4. Why was it important to examine the behavior of the hydrometer in distilled water? What did this part of the procedure tell you?
     
  5. Use your graph in Fig. 2.17 to determine the density of a liquid if the hydrometer reading is 23 mm, 47 mm, and 16 mm.
     
  6. How could you use the hydrometer and its standardization graph (Fig. 2.17) to determine the densities of solutions at different temperatures? Explain.
     
  7. List some other solutions you might want to test with a hydrometer. You can read the special feature on Hydrometers and Specific Gravity for ideas.
    1. Why would you want to test the density of solutions other than seawater?
    2. Explain how you could re-standardize your hydrometer for other kinds of liquid solutions.

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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.