TU-HHMI SCIENCE ACADEMY
C. M. Coyle
Update: October 13, 2008
CHEMISTRY MODULE #4 [3 groups (4 students each) = 12 total]
WHICH TOOTHPASTE WHITENS TEETH THE BEST?
5 dozen hard boiled eggs (1 day old)
1 pack of styrofoam cups (100)
5 lb bag of sugar
2 one quart plastic containers (for kool-aid)
1 pack of blue kool-aid
1 pack of cherry kool-aid
1 box of tea bags
1 box of instant coffee packets
2 rolls of wax paper
6 crest spin toothbrushes
12 toothbrushes (generic)
3 tubes of crest toothpaste
3 tubes of crest whitening toothpaste
3 tubes of colgate ultrabrite advanced whitening or simply white
3 tubes of generic whitening toothpaste (non-brand name)
Instructor Guidance: Which toothpaste whitens teeth the best?
For this particular module, the students will be divided into two groups to determine how
effective whitening toothpastes are in comparison to regular ‘non-whitening’
Initiate questions with the students regarding their understanding of toothpaste and how
it works? What are toothpastes made of? Are they toxic? Explain that the active
ingredient in toothpaste is fluoride which helps prevent cavities and can be very toxic is
swallowed. Why are their different flavors and types? What is the active chemical in
whitening toothpaste? Do they contain bleach? So, which whitening toothpaste is one
better and do they really work better than non-whitening toothpaste? Can we assume
that the best one is the most expensive one? Please have the students write their
predictions/hypotheses in their lab books prior to each set of experiments.
Do whitening toothpastes really whiten and do they work differently on different
types of stains?
Each group will test sets of regular and non-whitening toothpastes using hard-boiled
eggs that will be stained in coffee, tea, and Kool-aid to determine which factors lead to
the best whitening results.
Experiment 1: Which toothpaste works best on different stains?
Have some of the students prepare the Blue & Cherry Kool-aid, coffee, and tea for
staining the eggs, while the others start labeling the eggs by stain and toothpaste brand
to be analyzed. The students will need to prepare 5 eggs for each stain (20 eggs total).
Allow the eggs to sit in the different solutions for 15-20 minutes. During this time,
discuss with the students which variables need to be consider so that the results will be
comparable (amount of toothpaste, number of strokes, different people stroke at
different rates, amount of area on the egg being stroked, etc.). Remove the eggs (make
sure coffee and tea are cooled) and placed them on wax paper to dry. During this time,
have the students write down the active ingredients listed on each tube of toothpaste.
Once dry, they are now ready for analysis. Make sure the students record their method
[how much toothpaste? How many strokes (hard or soft)? How stained was the egg
before starting?]. This would be a good time to generate how they are going to gauge
the ‘whiteness’ of the egg. Have the students generate a ‘whitening gauge’ on the
board (scale of 1 – 10) for this purpose.
Once the experimental parameters are in place, make sure that the eggs are
appropriately labeled to avoid any confusion. To conserve the use of eggs, the students
will perform two experiments per egg – they will need to label each side of the egg using
a sharpee, clearly labeling what toothpaste is being used on each side of the egg. For
simplicity, have them write a line dividing the egg in half (length-wise) around the egg
(north to south poles). Have the students brush the stained eggs with the assigned
toothpaste (using a consistent amount of toothpaste) by holding the egg in one hand
(use gloves!) and brushing the egg with 100 strokes on the same spot. Have them
carefully rinse the eggs – try to avoid ‘rinsing’ the non-tested side if possible – and
record their observations based on the ‘whitening gauge’ they designed.
Have them perform this 3 times for comparison and have them record their
Experiment 2: Does the amount of foam represent the best cleaning?
Have the students reproduce the earlier experiment for each of the different stains using
the best ‘whitening’ toothpaste and least ‘whitening’ toothpaste determined in
experiment #1. How about it we used more of the least ‘whitening’ in comparison to the
normal amount for the best ‘whitening’ toothpaste? Holding all of the parameters equal
(area of brushing, 100 strokes, etc.), see if doubling the amount of the least ‘whitening’
to the standard amount of the best ‘whitening’ will make a difference? Have them
repeat this three times, while writing their observations.
Experiment 3: Does the amount and type of strokes better?
Using the best ‘whitening’ and least ‘whitening’ toothpastes, have the students conduct
a series of experiments on the different stains, but with increasing the number of strokes
for the least ‘whitening’ toothpaste (two times that of the ‘best’ whitening). Have the
students observe and record their results three times. Did this make a difference? Now
have the students reproduce this same experiment, except have ½ the students in each
group use the regular toothbrushes and the others use the spin toothbrushes provided.
Since we cannot count strokes on a spin brush, we will have to use what to compare?
Time! Have them conduct the experiment using the same amount of time (1-2 min) as
well as amount of toothpaste used. Have them repeat this three times, while writing
Did the most expensive whitening toothpaste really work the best? Did the
toothpaste that makes the most ‘foam’ whiten the best?
The students will now compare the data that they collected during the morning session.
Did the same toothpaste work best on all stains? Did the amount of toothpaste matter?
Which toothpaste worked the best and why? Have the students derive a final conclusion
of their project and what observations/comments can be made about whitening
The experiments should result in a moderate comparison of the whitening to non-
whitening toothpaste. Explain how some whitening toothpastes have hydrogen peroxide
in them. Some toothpastes claim they whiten teeth, but they actually have abrasives in
them that remove stains and plaque. Common abrasives include calcium phosphates,
alumina, calcium carbonate, and silica. We can demonstrate this reaction by the
The rapid catalyzed decomposition of hydrogen peroxide produces O2 gas that forms a
foam with the liquid detergent:
2H2O2 (aq) -> 2H2O + O2 (g)
The I-1 ion is a catalyst for the reaction. The brown color of the foam is evidence of iodine in the reaction
(it will stain clothes, skin, and carpet). Toothpaste should be abrasive enough to remove plaque and
stains, but not abrasive enough to damage tooth enamel. So, remember, the more foam doesn’t mean
the best toothpaste!