TU-HHMI SCIENCE ACADEMY
CHEMISTRY MODULES
FALL 2008
C. M. Coyle
Update: October 13, 2008
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CHEMISTRY MODULE #3
[3 groups (4/5/5 students each) = 14 total]
WHICH PROTECTS US BETTER FROM THE SUN – SUNBLOCK OR SUNSCREEN?
Materials:
4 packs of Sun Print Paper (30 sheets/pack)
3 dishpans for developing paper (shallow plastic bins)
2 gallons of distilled water
12 clear plastic report folders
12 scissors
2 tubes of sunblock (cheaper brand) at SPF 15, 30, 50
2 tubes of sunblock (expensive brand) at SPF 15, 30, 50
2 tubes of sunscreen (cheaper brand) at SPF 15, 30, 50
2 tubes of sunscreen (expensive brand) at SPF 15, 30, 50
2 cans of sunblock spray at SPF 15, 30, 45
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Instructor Guidance: Which protects us better from the sun – sunblock or
sunscreen?
For this particular module, the students will be divided into three groups to determine
the effectiveness of using different sunscreens, sunblocks, SPFs, and sprays vs. lotion.
Morning Session
Initiate questions Initiate questions with the students regarding their understanding how
sunblocks and sunscreens work? What is the difference between the two? Explain that
while sunscreens absorb UV rays, sunblocks reflect these rays. Currently, sunscreens
are more popular (available in creams, lotions and gels) and contain chemicals that
absorb UV rays. They are also inconspicuous on the skin. The chemicals that are most
often found in sunscreens include PABA, Benzophenones, Cinnamates, Salicylates,
On the other hand, sunblocks reflect UV rays and at one time were only available as
opaque white films/lotions/creams. However, new technology has created sunblocks
with particles so tiny, that the opaque film in no longer noticeable. Chemicals most
common in sunblocks include Titanium dioxide and zinc oxide.
So, is one better than the other? Also, what is SPF? Sun protection factor (SPF) does
SPF really matter? Do different brands provide better protection? Do the higher priced
sunscreens/sunblocks work better than cheaper ones? Please have the students write
their predictions/hypotheses in their lab books prior to each set of experiments.
Which sun protection works best?
Each group will experiment with a different component of the overall question
(sunscreen, sunblock, SPF, heat, moisture) to determine the best product for sun
protection.
All groups will perform the same experiments designed to not only discover the
effectiveness of sun protection, but the nature of sunlight and how light-sensitive
chemicals work. First explain to the students how the sun print paper works. This print
paper is coated with light-sensitive chemicals, which react to light waves and particles
when exposed to light. When you place objects or protective coatings on the paper, they
block the light exposure and turn white while the paper around them remains blue after
they are rinsed with water. Water stops the process and fixes your images on the paper.
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Please warn them of touching the paper with their fingers (moisture, acidity, etc.) will
affect their results.
Experiment 1:
Do sunblocks provide more protection than sunscreen?
Have the students cut four sheets of sun sensitive paper into four squares. Make sure to
label each of the squares: sunblock 1, 2, 3, and 4; sunblock (b) 1, 2, 3, and 4;
sunscreen 1, 2, 3, and 4; and sunscreen (b) 1, 2, 3, and 4. (b) will represent the
samples that will be placed in the binder. Remind them that there needs to be a ‘control’
in the experiment – this will be ‘1’ and will be left untreated (no sun product) to see how
the paper should look with no sun protection. Collect sunblocks and sunscreens (noncoppertone brand) at varying SPFs and spread (with gloved fingers!) one on each
square. Ask them if they need to change gloves every time? What about
contamination? Ask the students if the amount of lotion makes a difference? How about
the thickness of the lotion? What errors could occur if different people smear the
product onto the sheet? Warn them that they should attempt to apply the same amount
of lotion on each square making it as even as possible. Place the set of sunblock (1, 2,
3, and 4) and sunscreen (1,2, 3, and 4) ONTO one clear report folder each. You will
need one folder for the sunblock and another for the sunscreen samples. Now place
the set of sunblock (b) (1, 2, 3, and 4) and sunscreen (b) (1,2, 3, and 4) INTO one clear
report folder each. Why is this necessary? Remind them that this will prevent the lotions
from evaporating at different rates and to provide some ‘control’ to the experiment.
Have the students quickly take the treated squares outside where it was very sunny.
Allow the samples to sit outside for exactly 5 minutes. After the 5 minutes has passed,
have the students bring the samples back inside and immediately submerse the sun
paper samples into a white bin container (tray) that is ½ filled with distilled water. Allow
them to soak for 2 minutes and remove carefully onto paper towels to dry. While drying,
have the students note the active ingredients listed for the sunblock and sunscreen.
How are they different? The same? Once dry, the squares are ready for observation.
Have the students record their data and document their conclusions: is sunblock more
effective than sunscreen?
Experiment 2:
Does the amount of sunblock/sunscreen matter? How about in
spray form? How do these factors affect SPF (sun protection factor)? Does SPF
really matter?
Ask the students to compare the amount of sunblock/sunscreen used between the three
groups – are they the same? Were their results exactly the same? Do you think the
amount and the thickness is going to matter? Have the students repeat Experiment #1
exactly, except using a small amount (one drop) versus a larger amount (twice the
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amount of the smaller one drop) and spreading the lotion even across the square. Have
the students record their data and document their conclusions: does the amount of
sunblock/sunscreen affect their results from Experiment #1? Which one is really more
effective? Ask the students to determine the least amount of lotion required for effective
protection. Did the spray work as well as the lotions? Does SPF matter? The results
should show little change after SPF 15/30. Remind them that SPF matters (there is a
difference between the control and the SPF 15 sample), but the SPF number is less
important after SPF 15. Also, the spray mists the lotion in a more evenly distribution, but
not as thick. So, does that affect its sun protection capabilities? If so, how?
Experiment 3:
Is the more expensive sun protection the best?
Ask the students if they think price plays a role in sun protection? Have the students
repeat the same process using Coppertone products using the ideal amount of lotion
determined in Experiment #2. Did they see any advantages of using the more
expensive sunblock/sunscreen over the cheapest? Doe the SPFs protect to the same
degree regardless of price? What would you recommend buying to your parents and
friends?
Afternoon Session
Did the most expensive sun protectant work the best? Did the sunblock work
better than the sunscreen? Does it work in the shade?
The students will now compare the data that they collected during the morning session.
The experiments should result in a moderate comparison of the protection provided by
the lotions. The results will vary based on the handling of the higher light sensitivity of
the developing/photo paper. This can be used to discuss how different
chemicals/surfaces are more/less sensitive to light. Have them access for other sources
of error that may arise. Were they able to distribute the lotion on the squares evenly?
Does thickness matter for protection? Does the SPF really matter or does it just matter
‘how much’ and ‘how often’ you apply it to your skin? If so, what kind of texture (thick or
thin) did the most effect lotion have? Also, does the price of the lotion reflect the level of
sun protection?
Finally, does it matter if it is sunny or cloudy? Have the students repeat the same
procedure using the ‘best’ sun protection; however, they will place both sets of squares
into binders. One set will be placed in the sun and the other in the shade. Expose,
soak, and dry for observation. Did the shade make the results any different? Did the
squares with no lotion (control) in the shade show any sun damage?
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Have the students now compare this data to the morning experiments. Did this change
their initial predictions? What have they determined about UV-ray exposure? Does the
shade protect you? Did sunblock work as affectively as sunscreen?
To finish, remind them that sunscreens and sunblocks are not the same thing. While
sunblocks serve to reflect UV-rays, very often your sunscreen will contain chemicals
that will actually absorb the UV-rays and then scatters them. Also inform them that 80%
of lifetime exposure to sunlight occurs before the age 18. That means that for some of
us, it is too late to worry about the damage that has already taken place. The good
news is that they are still very young and have a far better chance of protecting
themselves from skin cancer, and guaranteeing great looking skin in the future. 
Now, allow the students (as a group) to cut three sheets of sun print paper into four
squares and allow each to make a design (ex. Happy face, name, etc.) using the
sunblock/sunscreen that they believe provides the best protection. Allow them to
expose and treat as previously. Once dry, the squares are ready for them to take home
in Ziploc bags. 
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