ACTIVITY: Reappearing Salt

The activity below draws from content in the page Matter in the Sea

 

Phenomenon:

If you don’t rinse off after swimming in the ocean, your skin will feel salty (Fig. 1)!

Inquiry:

If the ocean is salty, why can’t we see the salt?

Guiding Questions:

1. What is the difference between water from the ocean and water from a lake or pond?
2. How do we know the ocean is salty?

Activity:

Create your own saltwater solution by dissolving salt in water, then bring it back by evaporation.

Materials:

• Student worksheet and teacher guide (attached below):

  

• 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)  

Teacher Recommendations and notes: 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. Although the salt used in this activity will create saltwater, it is not the same water that can be used in a saltwater aquaria. 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. As a further investigation, provide students with seawater and/or water from a natural body of water to observe the variety of matter that appears after evaporation. Check out the example video at the bottom of the page to watch the activity Recovering Salts from Seawater.

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Procedure:

1. Follow along on your worksheet to make saltwater and answer questions.

Explore the Salt:

2. Lay out your black construction paper in front of you and put a teaspoon of salt onto it.
3. Look at the salt through a magnifying glass and draw or write down your observations and hypotheses on your worksheet.
   1. What do you see?
   2. What do you think will happen if you put the salt in the water?
   3. What do you think will happen if you evaporate the water from the glass?

Make a Large Batch of Saltwater:

4. Measure out 3 1/2 teaspoons of salt into a pitcher (about 20 grams).
5. Add 4 1/4 cups of water (about 1000 mL).
6. Stir the salt and water mixture.
7. If you are using food grade salt, taste the mixture using a clean spoon.

Evaporate Saltwater:

8. Pour small portions of your saltwater into your glass containers.
9. Evaporate the water:
   1. In the oven:
      1. Bake the samples in an oven on a baking sheet until all the liquid has evaporated.
      2. Keep the oven temperature under 95˚C (≈200˚F).
   2. With the sun:
      1. Place the samples on a window sill or somewhere with direct sunlight.
      2. Let the water evaporate naturally; this may take days to weeks, depending on the amount of water used.
      3. Revisit the cups regularly to observe the progress of evaporation.
   3. Using a heat lamp or hot plate:
      1. ​heat the samples to no more than 60˚C. Do not let the water boil.
      2. 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.
      3. If the crystals begin sputtering, turn down the heat.
10. When all of the water is gone, examine the salt crystals under a magnifying glass again. Write or draw your observations on your worksheet. Include a descriptive label that explains how you evaporated your water.

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Activity Questions

1. What happened when you added salt to the water?
2. Can you see the salt in the water after you stirred it?
3. Where does the ocean get its salt?
4. What does this activity tell you about the materials in the ocean?
5. You have just established that you can’t see salt when it’s dissolved in water, but you know that it’s there. Can you think of other examples of matter that you know is there but can’t see? (Hint: think about sugar, food coloring, or a fire)
6. How does this activity help you to understand that matter is too small to be seen?

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Further Investigation

1. Try this experiment using seawater collected from the ocean! Observe the variety of matter that appears.
   1. How does the salt from the ocean compare to the one you made from table salt?
2. Compare water collected and evaporated from other bodies of water.
   1. What do you notice?

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Example Activity:

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.