Expt 022 -- Goldenrod Messages

Description

Spray a solution on a piece of yellow paper, and a secret message instantly appears.

Chemical Concepts

  1. Indicators are dyes that change colors when their chemical environment changes.
  2. One very common type of indicator is an acid/base indicator which undergoes a very discrete color change when its environment surpasses a certain pH. Phenolphthalein and litmus are two common examples.
  3. The reaction that causes the color change is almost always reversible, and so the indicator changes back when its environment drops back below that same pH.
  4. Some industrial dyes are coincidental indicators -- such as the goldenrod dye used in some types of paper.
  5. Likes dissolve likes; thus, nonpolar wax molecules do not dissolve in polar water molecules.

Reactions

("HGr" = goldenrod indicator)

#1 inside the spray bottle: NH3 + H2O --> NH4+ + OH-
#2 on the sign: HGr (yellow) + OH- --> Gr- (red) + H2O
When you dip the paper in NaHCO3 solution, the reactions are:
#1 Dipping HGr (yellow) + HCO3- --> Gr- (red) + H2CO3
#2 Drying H2CO3 --> CO2 + H2O
Drying shifts the equilibrium above by removing CO2 and consequently, H2CO3 in the reaction above. More red Gr- forms on drying.
#3 On the sign: Gr- (red) + HC2H3O2 --> HGr (yellow) + C2H3O2-

Safety

Wear goggles.

Procedure

1. Before class, tape together 2 or 3 sheets of goldenrod paper, then flip them over so that the tape is on the back side.

2. Use a candle to write a hidden message (such as "Hi!", "Hello!", "WOW!!"... or if you are using it to introduce the topic: "Acids & Bases" or "Indicators!"). Or, use Scotch® brand, Magic® tape or any non-visible adhesive tape in place of the candle wax. Although it may take longer to write the messages with tape, the letters come out much cleaner and sharper. Tape the sign up on the chalkboard. Brush aside any pieces of paraffin that might reveal the letters: the message should be invisible!

!!!Click here to See Movie. Click |> or <| to step the slides forward or back.

3. Place some dilute ammonia solution in the spray bottle. Once the students have entered the classroom, spray the sign with the solution. The paper turns immediately red, except for where the words were written which remains bright yellow -- and the secret message becomes instantly visible. Then, as the ammonia diffuses out, the message fades and the sign dries -- just in time for you to re-use it for the next class!

!!!Click here to See Movie.

!!!Click here to See Movie. Click |> or <| to step the slides forward or back.

Variations:

  1. Use baking soda solution instead of ammonia. The message forms more slowly as the CO2 evolves from the solution leaving a basic solution behind. The message does not fade.
  2. Start off with goldenrod paper that has been soaked in 2% baking soda solution and dried -- so it starts off dark red. The red color darkens as it dries because some CO2 evolves leaving a more basic solution.
    !!!Click here to See Movie. Click |> or <| to step the slides forward or back.
    Make the sign and spray it with some vinegar!
    !!!Click here to See Movie. Click |> or <| to step the slides forward or back.
  3. Rather than making a message and spraying it, simply post up some goldenrod paper (and some pre-treated red goldenrod paper), then use a bundle of cotton swabs or Q-tips dipped in ammonia solution (or vinegar) to "paint" a message. This is a good elementary level art/science activity. It can also be used to for making colorful name-tags the first day of chemistry class!
  4. Use a box slightly smaller than your paper and a small screw cap glass bottle with a small neck (a small glass soda bottle works well). Open one end of the box by folding the flaps inward. Poke a hole into the center of the side of the box with a scissors. Widen the hole up by turning the scissors. The neck of the bottle must fit snugly into the hole. Write a message in wax or tape backwards (mirror writing) on a sheet of goldenrod, then tape it, message-side facing inward across the open end of the box. No marks are present on the outside of the paper.
    !!!Click here to See Movie. Click |> or <| to step the slides forward or back.
    Pour some concentrated ammonia solution (caution: wear goggles, good ventilation required) into the small glass bottle -- just enough to cover the bottom. Place the box over the neck of the bottle. The message appears soon (forwards, not backwards) as the ammonia diffuses up and dissolves in the moisture of the paper. Because it stays dry, the same sign can be used and re-used indefinitely.
    !!!Click here to See Movie. The movie is accelerated 5 times the actual speed.
    Remove the box and screw the bottle shut. Place the box so that the fading is visible to the students.

Questions

  1. Describe the chemical processes taking place when the message first appears.
  2. Write chemical reactions for the changes taking place when the message first appears.
  3. Describe the chemical processes taking place when the red color slowly fades back to yellow.
  4. Write chemical reactions for the changes taking place as the red slowly fades back to yellow.
  5. Describe procedures which can accelerate the process.

Handout Makeup

Name ___________________________ Class ________

Teacher __________________________

BeckerDemos 022 Goldenrod Messages

Watch the movies.

  1. Describe the changes in the paper.
  2. Answer the questions.

Curriculum-

Use this experiment to introduce acid-base indicators or reversible reactions of any kind.

Activity-

Demonstration - Student or Teacher

Safety-

Wear goggles. Follow routine safety precautions.

Time-

Teacher Preparation: 5 minutes

Class Time: 5 minutes

Materials-

Optional:

Disposal-

Dispose of liquids at the sink.

Lab Hints-

Observations-

Reactions: ("HGr" = goldenrod indicator)

#1 inside the spray bottle: NH3 + H2O --> NH4+ + OH-
#2 on the sign: HGr (yellow) + OH- --> Gr- (red) + H2O

Then, as the NH3 diffuses out of solution in rxn #1 above, the equilibrium is shifted to the left, using up the OH- ions. As the OH- concentration decreases, the equilibrium in rxn #2 shifts to the left restoring the original yellow color.

Along with illustrating the concepts of acid/base chemistry, indicators and diffusion, these messages also demonstrate the idea 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.

Answers-

Q1. Describe the chemical processes taking place when the message first appears.
A1. The basic ammonia solution is reacting with the indicator to change it from yellow to red, but only where the paper is not shielded by the nonpolar wax (or tape).
Q2. Write chemical reactions for the changes taking place when the message first appears.
A2. NH3 + H2O --> NH4+ + OH-
HGr (yellow) + OH- --> Gr- (red) + H2O
Q3. Describe the chemical processes taking place when the red color slowly fades back to yellow.
A3. The ammonia is diffusing into the air. The diffusion shifts the pH back to neutral.
Q4. Write chemical reactions for the changes taking place as the red slowly fades back to yellow.
A4. Gr- (red) + H2O --> HGr (yellow) + OH-
NH4+ + OH- --> NH3 + H2O
Q5. Describe procedures which can accelerate the process.
A5. Anything, which would accelerate the evaporation or change the pH, would work here. For example: heat the paper; blow air over the paper; or spray with vinegar.

Reference-

Acknowledgments: Marie Sherman of St. Louis, MO, first showed me how goldenrod paper acts as an indicator.

Key Words 1-

acid, base, indicator, reversible reaction, pH