Unit 5: Chemical Reactions
Acids & Bases: Invisible Ink Lab
Name _______________________________________________
Date ________________ Period _______ Assignment 12
Invisible Ink Lab
Introduction:
Invisible ink has been used for many applications such as espionage, counterfeit money, and magic coloring books for kids. The ink is
applied to paper and upon drying becomes invisible. The ink becomes visible again by applying chemicals. Some chemicals that are
used for invisible ink involve acids and bases. Acids produce an H+ ion in solution and bases produce an OH- ion in solution. Acids
and bases have different effects on acid-base indicators. There are many different kinds of indicators that each change color
depending on pH, which measures how strong an acidic or basic solution is. For example, litmus paper turns red for strong acids and
turns blue for strong bases, but turns green for neutral solutions such as water (pH = 7). We will make invisible ink using a simple
acid-base indicator reaction.
We will use phenolphthalein, which is colorless in an acidic liquid but turns pink in a basic liquid. The pH change which causes the
color to disappear is a result of the reaction between basic sodium hydroxide, NaOH, and acidic carbon dioxide, CO2.
Materials:
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Safety goggles
Latex gloves
2 plastic cups
1 permanent marker
100 mL graduated cylinder
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10 mL Phenolphthalein solution
90 mL water
1 pipette dropper
NaOH solution
1 popsicle stick
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4 Q-tips
4 blank pieces of paper
1 timer
Acids: vinegar, lemon juice
Bases: ammonia, windex, soapy water
Procedure:
 1. Put on safety goggles & latex gloves.
 2. Label 1 cup ‘NaOH’ & label 1 cup ‘Phenolphthalein.’
 3. Use the graduated cylinder to measure 10 mL phenolphthalein solution & add it to the ‘Phenolphthalein’ cup.
 4. Use the graduated cylinder to measure 90 mL of water, add it to the ‘Phenolphthalein’ cup & use a popsicle stick to stir.
 5. Use the pipette dropper to add NaOH drop by drop to the ‘Phenolphthalein’ cup. Stop just when the mixture turns blue.
 6. Use a Q-tip and the phenolphthalein solution to write the following message on all 4 pieces of paper at the same time:
Acids have pH 1 – 7. Bases have pH 7 – 14.
 7. Use the stopwatch to measure how long it takes for the last letter to disappear with the application an acidic solution
and record your data in the table below.
 8. Use the stopwatch to measure how long it takes for the last letter to reappear with the application a basic solution and
record your data in the table below.
Data:
Time to Disappear
Acidic Solution
Time (m:ss)
None
CO2 from Exhaled Breath
Vinegar
Lemon Juice
Time to Reappear
Basic Solution
Time (mm:ss)
NaOH
Ammonia
Windex
Soapy Water
Conclusions:
1. What causes the color to disappear? What causes the color to reappear?
2. Would you expect the color to reappear if sprayed again?
3. Which acid was strongest? Which base was strongest?
4. Use at least 1 complete sentence to explain how phenolphthalein solution acts as an acid-base indicator.
5. How could an acid-base indicator be used to help someone who has trouble digesting acidic food?