Rainbow Cups
Introduction to the Scientific Method
This is a good “first day” demonstration.
Although it seems a bit “magical” at first, it can serve as an introduction to the scientific method
Chemicals and Equipment Needed
 Indicators – H5- small drop bottles in baggie
o phenolphthalein, thymolphthalein, p-nitrophenol
 Very dilute NaOH solution – H5
 6-300 mL tall beakers – Q2
 6 plastic petri dish lids – P3
 Light box – A4
Preparation
 Label the beakers under the rim, 1-6
 Right before class/delivery, add drops of indicator according to chart below:
You have to use the drop bottles labeled for Rainbow Cups.
BEAKER
phenolphthalein
thymolphthalein
p-nitrophenol
color
1
3
1
R
2
1
4
O
3
4
5
3
2
Y
2
4
G
6
3
2
B
V
Presentation
In introducing the scientific method, we talk about defining a problem, formulating a hypothesis, making
predictions, testing the hypothesis, and so on. In the demonstration, you will add NaOH solution to the
beakers one at a time. Let the students choose which cup you should fill first and ask them to predict what will
happen. Call on another student to choose another cup and predict the result. How long does it take the class
to discover the pattern and correctly predict the results?
In recognizing the rainbow pattern and using it to predict results for other cups, the students are
drawing on knowledge and experience they might not perceive as “scientific.” But such knowledge and
experience is just as important to scientists as they work through problem-solving.
Once the students have “broken the code” and you have filled 5 of the 6 cups, you can make an
important point. Instead of filling the sixth cup, say, “We don’t need to fill this one, we know what color it will
be.” Hopefully the students will protest and insist that you fill it to make sure. The point to make is that
scientists must always test their predictions and verify the results- the hypothesis must be reformulated if any
flaws are discovered.
Clean-Up
 Solutions can go down the drain with plenty of water
Chemical notes: 0.02M NaOH- 6.5mL 3M NaOH diluted to 1L solution, or 6 NaOH pellets in 1L; indicators(mass in 100mL ethanol)- phenolphthalein- 1.5g; thymolphthalein- 2.0g; p-nitrophenol- 10g
Acknowledgment: This demo was learned from George Gilbert of Denison University
Revised Spring 2015
AMM