Introduce acids and bases by revealing a hidden message by spraying goldenrod paper with a mysterious liquid!
- Acids and bases
Ammonia, clear household
Beaker, 1 L
Goldenrod paper, 2-3 sheets*
White candle or clear tape
*Materials included in kit.
Household ammonia solutions are slightly toxic by ingestion and inhalation; both liquid and vapor are extremely irritating, especially to the eyes. Wear chemical splash goggles and use in a well-ventilated room. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All the solutions may be reused and the goldenrod paper can be either reused or disposed of in the trash.
- Before class, attach 2–3 sheets of goldenrod paper with tape and flip them so the tape is on the back side.
- Use candle wax or clear tape to write a hidden message, and tape the sign up on the board or wall. Brush aside any pieces of paraffin that might reveal the letters: the message should be invisible!
- Place the household ammonia solution in the spray bottle. Spray the paper with the solution. The paper will immediately turn red, except for where the words were written, which will remain bright yellow—and the secret message will be revealed.
- Alternatively, half-fill a large beaker with ammonia. Tell your students that a new liquid has been discovered that can pull blood directly from a person’s hand. Dip your hand into the beaker and then go to where the paper is hanging and shake your hand at the paper. Your students will be shocked as the paper turns red and the “blood” spatters. Have another paper at the ready with a hidden "Hello!” or “Gotcha!” message (see Figure 1) that you can spray with the ammonia in the bottle.
- As the ammonia diffuses out due to the carbon dioxide in the air and the paper dries, the message will fade—just in time for you to reuse it for the next class!
- Another option is to start by soaking the goldenrod paper in 2% baking soda solution and letting it dry; it will remain dark red. Write a hidden message as before, but this time spray the paper with vinegar. The message is revealed in bright yellow.
- All the solutions may be reused, and the godlenrod paper can be reused or disposed of in the trash.
Answers to Questions
- Describe what happened in this demonstration.
A piece of yellow paper was sprayed with ammonia, and the paper turned red, except for a message that had been written on the side with wax. Eventually the paper fades back to yellow.
- The solution in the spray bottle is ammonia.
- Write an equation for the reaction of ammonia with water.
- Is ammonia an acid or a base?
Ammonia is a base.
- Goldenrod paper contains an acid–base indicator. What is the color of this indicator in acid? What is the color of this indicator in base?
The goldenrod indicator is yellow in an acid and red in a base.
- Why does the sign eventually turn yellow again?
NH3 molecules on the sign diffuse out of solution and evaporate, and equilibrium shifts to the left. This uses up the OH– ions and, as the concentration of OH– decreases, the color fades back to yellow.
Reactions: (“HGr” = goldenrod indicator)
- Inside the spray bottle and on the paper:
- Color change on the sign:
The first reaction shows how ammonia reacts as a base with water, producing OH–
ions. The second reaction accounts for the dramatic color change as the OH–
ions react with the indicator. Then, as NH3
molecules on the sign diffuse out of solution and evaporate, the equilibrium is shifted to the left in reaction 1, using up the OH–
ions. As the OH–
concentration decreases, the equilibrium in reaction 2 also shifts to the left restoring the original yellow color.
Along with illustrating the concepts of acid–base chemistry, indicators, diffusion and equilibrium shifts, these messages also demonstrate the phenomenon of selective solubility; for although the polar ammonia molecules dissolve quite readily in the polar water molecules, neither of the two mixes well with the nonpolar wax molecules and hence, the indicator beneath the wax remains yellow.