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Activity: Recovering Salts From Seawater
NGSS Science and Engineering Practices
NGSS Crosscutting Concepts


  • Seawater
  • Fresh water
  • 10 mL graduated cylinder
  • Two watch glasses
  • Permanent marker
  • Oven (or sun, heat lamp, or hot plate)
  • Oven mitts
  • Baking sheet
  • Balance
  • Dissecting microscope
  • Table 2.1
  • Teaspoon (optional)
  • Food-safe cups (optional)
  • Toothpicks (optional)


Safety Note: If tasting samples at the end, use a food-safe graduated cylinder and watch glasses, or use a food-safe teaspoon and cups. Food-safe means items that have not been used with laboratory chemical or biological substances. Use heat resistant gloves or tongs if using the oven or hot plate to evaporate water from your samples.

  1. Determine the masses of 10 mL samples of seawater and fresh water. (Or, if tasting samples at end, measure two teaspoons of each liquid into food-safe cups.)
    1. Label one watch glass as “seawater” and the other watch glass as “fresh water” with a permanent marker.
    2. Weigh one of the watch glasses. Record the mass in Table 2.1. 
    3. Using a graduated cylinder, measure a 10 mL sample of seawater and pour it into the watch glass.
    4. Weigh the watch glass and the seawater. Record in Table 2.1.
    5. Subtract the mass of the watch glass from the mass of the watch glass and seawater together to find the mass of the seawater. Record the result in Table 2.1.
    6. Repeat steps b–e with fresh water.
  2. Evaporate the water from the samples.
    1. Bake the samples in an oven on a baking sheet until all the liquid has evaporated. Keep the oven temperature under 95˚C (≈200˚F). 
    2. Alternately, evaporate the liquid using the sun, a heat lamp, or a hot plate. To use a hot plate, heat the watch glass to no more than 60˚C. Do not let the water boil. When the water is almost gone and the crystals look slightly wet, you can remove the remaining water by turning up the heat a little until the crystals are dry. If the crystals begin sputtering, turn down the heat.
  3. Find the mass of the crystals in the samples.
    1. Weigh the seawater watch glass and dry crystals. Record the result in Table 2.1.
    2. Subtract the mass of the watch glass from the mass of the watch glass plus the salt crystals. Record your results for the mass of crystals in Table 2.1.
    3. Repeat steps a¬–b with the fresh water watch glass.
  4. (Optional) Check to ensure the crystals are dry. This step is recommended if you evaporated the liquid using the sun.
    1. Return the watch glasses to the sunlight for another day.
    2. Weigh the watch glasses and dry crystals. Subtract the mass of each watch glass from the mass of the watch glass plus the salt crystals.
    3. Compare the mass of the crystals obtained with the mass of crystals obtained in procedure 3.
    4. If the mass of the crystals is less than the mass of the crystals in procedure 3, repeat steps a–c until the mass does not change.
  5. Look at the crystals under a dissecting microscope and describe them.
    1. Draw the shapes of all of the crystals you see in each of your samples. 
    2. Describe the location of the different types of crystals.
    3. Record any other observations.
  6. (Optional) Taste each type of crystal you observe. Record how each type of crystal tastes. If sharing watch glasses, use toothpicks to pick up the crystals to taste them.


Activity Questions
  1. Where do the salt crystals come from? How do you know?
  2. Describe the crystals that were formed from the evaporation of seawater and fresh water.
    1. How many kinds of crystals did there appear to be in each dish?  
    2. How were the crystals distributed? 
    3. What is the evidence for your answers?
  3. What happened to the mass of the samples after heating? Why?
  4. How might your results have been affected if you did not ensure your crystals were a constant mass (procedure 4)?
  5. What might have happened if the samples had boiled during evaporation?  How would this have affected the results?
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.