SNC2D/2P Chemical Reactions/Chemical Reactions and their Practical
Applications
Teacher Demo: Invisible Ink
Topics
Timing
properties of acids and bases
pH scale
preparation: 10 min
demonstration: 10 min
Specific Expectations
SNC2D
A1.1 formulate scientific questions about observed relationships, ideas, problems, and/or issues,
make predictions, and/or formulate hypotheses to focus inquiries or research
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions
C3.7 describe how the pH scale is used to classify solutions as acidic, basic, or neutral (e.g., a
solution with a pH of 1 is highly acidic; a solution with a pH of 7 is neutral)
SNC2P
A1.1 formulate scientific questions about observed relationships, ideas, problems, and/or issues,
make predictions, and/or formulate hypotheses to focus inquiries or research
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions
C3.5 describe how the pH scale is used to identify the concentration of acids and bases
Introduction
In this demonstration, a chemical reaction is used to make a hidden message visible. The
demonstration illustrates an entertaining application of chemical reactions in toys and everyday
objects. It is also an interesting way to start the discussion of acids and bases, the pH scale, or
acid–base indicators. Invisible ink has been used in espionage to send messages that cannot
easily be read by enemies.
Materials
chemical safety goggles
lab coat or apron
protective gloves
vinegar (5% acetic acid, HC2H3O2 (aq))
window cleaner (with ammonia) or household
ammonia
small bottle of phenolphthalein solution
cotton swabs or thin paint brush
paper towels or plain paper
3 spray bottle(s)
tape
Safety Considerations

Provide MSDS sheets for all chemicals used.



Window cleaner and vinegar should not come into contact with eyes. If this occurs, flush
eyes with water.
Do not inhale chemicals directly. Ensure that the laboratory is well ventilated.
The phenolphthalein solution is flammable. Keep it away from any heat sources and open
flames.
Hazardous Materials Identification System Rating
(0-minimal 1-slight 2-moderate 3-serious 4-severe)
phenolphthalein pH
window cleaner (with vinegar (4-6% acetic
indicator solution
ammonia)
acid)
household ammonia
(if used instead of
window cleaner)
Procedure
Wear proper PPE: chemical safety goggles, lab coat or apron, and protective gloves.
Prepare three spray bottles before class:
1.
Fill one spray bottle with vinegar and label it “A”.
2.
Fill another bottle with a window cleaner (containing ammonia, NH3(aq)) and label it “B”.
3.
Fill the third bottle with a mixture of window cleaner and vinegar. Label it “A + B”.
In class:
4.
Show students the three bottles. Explain that bottle “A” contains an acidic solution, bottle
“B” contains a basic solution, and bottle “A + B” contains a mixture of the acidic and basic
solutions.
5.
Dip the cotton swab or paint brush in the phenolphthalein solution and write a message or
draw a picture on the paper towel or blank paper. Do this to two more pieces of paper
towel. Allow them to dry completely (at least 15 minutes). Optional: Tape them on a wall.
6.
Spray the first paper towel using spray bottle “A” and have students record observations on
what they see.
7.
Predict/Explain
Ask students to predict what will happen when the remaining two paper towels are
sprayed. Students should also attempt to come up with a rationale for their predictions.
8.
Observe
Repeat Step 5 twice using a different paper towel and different spray bottle each time.
Provide time for students to record their observations.
9.
Explain
Ask students to explain their observations.
Disposal
The paper towels can be disposed of in the garbage. The window cleaner solutions can be stored
and used for future demonstrations or for actually cleaning window surfaces. Follow disposal
procedures that are consistent with school board protocol and appropriate for your municipality.
What happens?
No colour change is observed when the first paper towel is sprayed with vinegar. When the
second paper towel is sprayed with window cleaner, the message/picture appears in pink. As the
paper dries, its message slowly fades and disappears. No message is observed when the third
paper towel is sprayed with a mixture of window cleaner and vinegar.
How does it work?
An acid–base indicator is a substance that changes colour as a result of a change in pH. For
example, phenolphthalein is colourless below pH 8.2 and pink above pH 8.2. The ammoniabased window cleaner is basic due to release of hydroxide ions from the following reaction.
NH3(g) + H2O(l) → NH4+(aq) + OH–(aq)
ammonia
The presence of hydroxide ions results in phenolphthalein changing colour from colourless to
pink. The message slowly fades because carbon dioxide in the air dissolves in the solution to
produce carbonic acid.
H2O(l) + CO2(g) → H2CO3(aq)
No colour change is observed when the mixture of vinegar and window cleaner is used because
the window cleaner is neutralized by the acetic acid in vinegar:
HC2H3O2(aq) + NH4OH(aq)
→ C2H3O2NH4 (aq)+ H2O(l)
acetic acid
ammonium hydroxide
Teaching Suggestions/Hints
1.
If you tape the paper towels sheets to a wall or surface, make sure it is a surface that will
not be damaged by window cleaner.
2.
The same paper towel with its invisible message may be re-used several times.
3.
A demonstration that illustrates similar concepts is disappearing (instead of invisible) ink.
See additional resources for link to instructions.
Next Steps
This demonstration could be used as a starting point for students to investigate and come up with
their own version of either invisible or disappearing ink using various acid–base indicators and
other household materials (e.g., lemon juice, honey, milk, or soapy water with heat; corn starch
with iodine).
Additional Resources
1.
2.
3.
An alternative demonstration of disappearing ink (starting off blue and then slowly
disappearing) highlights similar concepts.
http://www.stevespanglerscience.com/experiment/vanishing-ink
More information about phenolphthalein:
http://www.chemistrydaily.com/chemistry/Phenolphthalein
More information about other acid–base indicators:
http://www.chemguide.co.uk/physical/acidbaseeqia/indicators.html