Colourful Chemistry
Submitted By: Julie
Overview
The goal of this project is to make solutions that can change colours when we speak or blow into
them, as well as solutions that change colours when they are swirled or shaken, using pH and
redox indicators.
Topic Area(s)
Chemistry
Age
8 – 10
Cost
?
Time
Prep: 20 min
Execution: 80 min
Clean-up: 20 min
Complexity
Preparation= 3
Execution= 2
Objective
How indicators
work to show
acid or base
solutions.
Hook
The chemical solutions we mix will be able to change colour when you want them to, without having
to actually add a different colour!
Necessary Background Information
To differentiate between acids, like vinegar, and bases, like soap, people use the pH scale. The
scale ranges from 1-14; acids range from 1 to 7, with acids closest to 1 being the strongest or most
acidic. Bases range from 7 to 14, with 14 being the strongest or most basic. Neutral solutions, like
water, have a pH of 7. A pH indicator is a chemical compound that is added in small amounts to
a solution so that the pH of the solution can be determined visually by a colour change. A pH
indicator is actually a chemical detector for hydronium (H3O+) or hydrogen ions (H+), which have
higher concentrations in acidic solutions. Depending on the pH, a solution will be of a certain colour,
for example, phenolphthalein is clear in a solution where the pH is below 8.2, but turns pink when
the pH of the solution reaches or surpasses that threshold, becoming more basic.
The first solution created uses the concept of a pH indicator to cause the colour change.
Bromthymol blue is blue at a pH above 7.6, a green-ish colour in a neutral solution, and becomes
yellow in an acidic solution, when the pH is below 6. The CO2 being introduced into the solution
when someone speaks (or breathes) into it reacts with water to produce hydrogen ions in the
reaction:
CO2 (g) + H2O (l)  H2CO3 (aq)  H+ (aq) + HCO3- (aq)
This causes the pH to become lower and lower, until finally it is below 6, causing the indicator to
turn the solution yellow. The students need to speak or breathe into the solution many times to
introduce enough CO2 to change the pH significantly.
The second colour change is based on a redox indicator, which undergoes a colour change at a
specific electrode potential, so it indicates if a solution is in a reduced state or an oxidized state.
Indigo carmine is a pH indicator, turning yellow in basic solutions, as well as a redox indicator. The
solution used is basic and reduced, so it is yellow in colour initially. Swirling the Erlenmeyer causes
oxygen to dissolve into the solution, which oxidizes it, causing the colour to change to red. Shaking
more vigorously causes even more oxygen to dissolve into the solution, making it more oxidized
and causing the colour change to green. When the solution is left to sit, the amount of oxygen in
the solution diminishes as the solution is reduced, causing it to change back to red and finally to
yellow once it comes back to its initial state. Swirling and shaking again causes the oxidation to
reoccur, inducing another colour change to red, then green, and so on.
Materials
For the voice activated colour change (students in groups of 2):
 250 mL water
 A few drops NaOH
 Erlenmeyer flask with a stopper
 A few drops bromthymol blue or phenolphthalein
For the stoplight colour change (groups of 2):
 3g dextrose
 5g NaOH
 350 mL water
 0.1 g indigo carmine powder
 Erlenmeyer flask with a stopper (same one as in the first reaction)
 Beaker
Safety Considerations
Make sure everyone washes their hands after working in the lab. Be careful not to get any solutions
in your eyes or mouth. Follow all laboratory rules; shoes, lab coat and goggles must be worn, no
food or drink in the lab, no running in the lab.
Procedure
Voice-activated colour change:
1. Pour 250 mL of water into the Erlenmeyer flask.
2. Add 1-2 drops of NaOH.
3. Add 5-6 drops of bromthymol blue or phenolphthalein.
4. Stopper the flask.
5. First person opens the flask, and speaks or blows into it.
6. Put the stopper back on the flask.
7. Swirl a little and pass to your partner.
8. Partner opens the flask, and speaks or blows into it.
9. Put the stopper back on the flask.
10. Swirl a little and pass back to the first partner.
11. Continue talking or blowing into the solution until it changes colours! This should take 10-15
turns.
12. Once the reaction is complete, rinse out the Erlenmeyer so it can be reused for the stoplight
reaction.
Stoplight reaction:
1. **BEFORE STARTING THE PROJECT, make an indigo carmine solution using the 0.1 g of
indigo carmine and 100 mL of water. This should be enough for all the groups.
2. Place the 3 g of dextrose and 5 g of NaOH into the beaker.
3. Add 250 mL of water.
4. Stir the solution until all the solid is dissolved.
5. Place 100 mL of the solution into the flask.
6. Fill a plastic pipette with the carmine blue solution and put it into the Erlenmeyer.
7. Repeat step 6 (you should have two pipettes-full of indigo carmine liquid in the solution).
8. Stopper the flask.
9. Let the solution sit for a few minutes.
10. Observe the colour (after sitting it should be a yellow colour).
11. Swirl the solution and see what happens (should turn red).
12. Shake the solution and see what happens (should turn green).
13. Let the solution sit again, and see what happens.