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ACTIVITY: Reappearing Salt

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

Phenomenon:

If you don't rinse off after swimming in the ocean, your skin is salty.


Inquiry:

Where does the salt come from if we can't see it?


Activity:

Create your own saltwater solution and then bring the salt back.


Guiding Questions:

  1. What is the difference between ocean water and fresh water?

Materials:

<p>Fig. 1. Some of the materials needed to make a small scale model of seawater.</p>

  • Table Salt
  • Teaspoon
  • Measuring cup
  • Pitcher (or cups if making smaller portions)
  • Watch glasses (or small glass containers that are oven-safe)
  • Water
  • Stir stick
  • Magnifying Glass
  • Black construction paper
  • Oven (or sun, heat lamp, or hot plate)  

Recommendations:

  • Safety Note: If tasting samples at the end, 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.
  • Depending on how you plan to evaporate your water, i.e. by sun or oven, it will take more or less time. Keep this in mind when deciding how much salt water to make. 
  • The instructions below make a larger pitcher of saltwater that can then be distributed to smaller student groups to evaporate. Or, you can make smaller portions (i.e. 1 3/4 teaspoons salt to 2 1/8 cups of water) so each group can make their own saltwater to evaporate. 

Salinity Background:

Salt is a mineral made up of sodium (Na) and chloride (Cl), forming the compound NaCl. We often refer to the amount of salt (or salinity) in the ocean as how much salt is in 1,000 g of water, or 'parts per thousand' (ppt). The average ocean has a salinity about 35 ppt. The salinity around the globe changes depending on other factors such as freshwater input or runoff from land (Fig. 2.). Input from various sources increases the amount of dissolved matter in the ocean and can influence the salinity.

<p>Fig. 2. Salinity in the ocean varies by area, ranging from 31 to 39 ppt, but the average is 35 ppt. Data shown here represent the sea surface salinity in 2009 and are from the World Ocean Atlas. The units shown here, Practical Salinity Units (PSU), represent the same information as Parts Per Thousand (ppt).&nbsp;</p><br />

Although the salt used in this activity will create saltwater, it is not the same water that can be used in a saltwater aquaria. This is because, although seawater is primarily a solution of sodium chloride (NaCl) in water, there are several other dissolved compounds in seawater that are important for living organisms. Because table salt does not contain these additional dissolved substances, it cannot be used to create a saltwater aquarium. Commercial sea salt formulated for aquariums and sold at pet supply stores contains trace elements in concentrations consistent with natural seawater and therefore can be used in saltwater aquaria.


Procedure:

Make Saltwater:

  1. Put a small scoop of salt (no more than a teaspoon) onto your peice of black construction paper.
  2. Take a look at the salt through a magnifying glass and draw or write down your observations. 
    What do you see?
  3. Develop your hypotheses:
    If I put the salt in a glass water, then ____________________________ because ______________________.
    If I evaporate the water from the glass, then _______________________because _______________________.
  4. Draw a diagram to model what you think will happen in the scenarios above.
  5. Measure out 3.5 teaspoons of salt into a pitcher.
  6. Add 4 1/4 cups of water (1000 mL).
  7. Stir the salt and water mixture.
  8. If using food grade salt, taste the mixture using a clean spoon.

Evaporate Saltwater:

<p>Fig. 3. After all of the water is evaporated, the salt reappears.</p>

  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 a heat lamp or hot plate.
    Note: 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. If using the sun, place the watch glasses or small cups on a window sill or somewhere out of the way with direct sun access.
  4. Let the water evaporate naturally, this may take weeks depending on the amount of water used. 
  5. Revisit the cups regularly to note the progress.
  6. When all of the water is gone, examine the salt crystals under a magnifying glass and write or draw your observations.

 


Activity Questions

  1. What happened when you added salt to the water?
  2. Can you see the salt in the water when you stirred it?
  3. Where does the ocean get its salt?
  4. What does this activity tell you about the materials in the ocean?

Example Activity: 

Recovering Salts from Seawater

Check out the video below to watch the activity "Recovering Salts from Seawater" demonstrated with teachers during a Teaching Science as Inquiry (TSI) workshop. This activity corresponds to high school content, so goes deeper into masses of salts and seawater than is needed at this grade level. 

">https://www.youtube.com/watch?v=Ln94New9ue8&feature=youtu.be]

Table of Contents:

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.