Kids, now that summer is upon us would you like a way to prove that a sunscreen works without using your own skin as the test? For this activity you will need a sheet of black & white newspaper or construction paper (red or dark blue work best), four zip-seal sandwich bags, two sunscreens (one lotion and one spray, both labeled “clear”, and both having the same sun protection factor - SPF), a piece of fabric, and a sunny outdoor location.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
Cut four pieces from the newspaper or construction paper and the fabric, sized to perfectly fit inside the plastic bags (about 5” x 6”). Place one piece of paper in each plastic bag so that the paper lies flat. Treat the bags as follows:
- Bag #1. Coat one side of the bag with a thin layer of sunscreen lotion.
- Bag #2. Coat one side of the bag with a layer of spray sunscreen.
- Bag #3. Place the fabric in the bag to cover the paper and lie flat.
- Bag #4. Leave as the untreated control.
Seal each bag. Place the four bags in a sunny location outdoors with the treated side up. You may need to anchor them so they remain flat and don’t blow away. After 1 or 2 sunny days, open the bags to observe the paper.
The paper in the untreated bag should fade or turn yellow (newspaper) after sun exposure. But the paper samples underneath layers of sunscreen remain protected from the sun’s rays and should retain their original color. The paper under the fabric should be somewhat affected; the degree depends on the fabric used. For example the SPF of white cotton is much lower than that of denim.
Sunscreens work either by absorbing or by scattering UV rays. Sunscreens that absorb UV rays contain organic molecules, usually octinoxate and/or avobenzone. Sunscreens that scatter UV rays contain inorganic compounds such as titanium dioxide (TiO2) or zinc oxide (ZnO). The smaller the inorganic particles are, the more transparent or “clear” is this type of sunscreen.
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
June 2010
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References: Erica Jacobsen, ACS ChemMatters, April 2010, page 15. For an explanation of sunscreen ingredients and for further activities see the same publication, page 13 by Gail Mitchell Emilsson.
