Image by Daniele Levis Pelusi

Salt Water Rainbows

Salt Water Rainbows 

by Sanda Sun,Ph.D.,

Professor of Chemistry at Irvine Valley College in Irvine, California.

Introduction

The amount of salt in a body of water affects its density. In fact, some bodies of water, like the Great Salt Lake and the Dead Sea, contain huge amounts of salt, allowing people to easily float on the water. How can you tell when one body of water will be denser than another? Follow these instructions to compare the densities of different salt water solutions.

 

Safety

✓ Safety goggles required
✓ Do not eat or drink any of the materials used in this activity

✓ Thoroughly wash hands after this activity
✓ Wear gloves to prevent stain on hands
✓ Cover your work area with newspaper or similar material to absorb any spills
✓ Disposal: All solutions can be safely poured down the drain.

Solid materials should be disposed of in the trash.

 

Materials

• 7 teaspoons (about 35 mL) salt • 1 quart (about 1 liter) water
• 1 permanent marker
• 1 teaspoon

• 4 stir sticks or plastic spoons
• 1 set of 4 assorted food colorings (yellow, blue, green, and red)
• 4 disposal pipettes OR clear plastic drinking straws
• 1 small test tube OR centrifuge tube OR narrow jar**
• 4 8-ounce (about 235 mL) foam or plastic cups

 

Procedures

  1. Label your 4 cups “0,” “1,” “2,” and “3” with the marking pen.

  2. Fill each cup half-full of water.

  3. Add 5 drops of yellow food coloring in cup “0.”

  4. Add 5 drops of blue food coloring in cup “1.”

  5. Add 5 drops of green food coloring in cup “2.”

  6. Add 5 drops of red food coloring in cup “3.”

  7. Dissolve 1 teaspoon of salt in the blue water in the cup marked “1.”

  8. Dissolve 2 teaspoons of salt in the green water in the cup marked “2.”

  9. Dissolve 3 teaspoons of salt in the red water in the cup marked “3.

  10.  Using a pipette, transfer 1/2-inch of the red salt water solution (from the cup marked “3) to the test tube/capillary tube/ narrow jar (whichever you are using).*

  11. Repeat step 10 with the green, blue, and yellow solutions.

  12. Draw a picture of your results.

  13. What would happen if you repeated the experiment but put the colored salt water in a different order? Try it!

 

NOTES

*If you do NOT have a pipette and are using straws, for step 10, dip the first straw 1/2-inch into the red salt water solution in the cup marked “3, then placing your index finger firmly on the other end of the straw, remove the straw from the solution. Put the straw over the test tube or narrow jar and remove your finger from the straw to release the water into the test tube. Then repeat with the green, blue, and yellow solutions.


**If a narrow jar is used, hold it at a slant and pour the solutions down the side of jar to avoid mixing.

 

How does it work? Where’s the chemistry?

The density of the solution increases as the amount of salt increases. The denser solutions stay on the bottom, and the less dense solutions float on top of solutions of greater density. The result is a four-colored rainbow! The rainbow effect would not form if the order were reversed. The denser solution would fall downward and contaminate the less dense solution.