Rainbow Connection: Rainbow Colors with Mixed Acid-Base Indicators

Brief Description: 

When a colorless liquid is poured into six beakers, the liquid in each beaker turns a different color of the rainbow.  The colors disappear and reappear when other colorless liquids are added to the beakers.  The colored solution beakers are then mixed in a large container resulting in the colors disappearing once again.

Purpose/Goal: 

A number of objectives are satisfied with this presentation, including a) demonstrating color changes of solutions due to changes in pH of the solutions, b) demonstrating the reversibility of indicator color changes, c) providing an introduction to acid/base chemistry, d) getting students to think about how acids and bases interact, and e) providing an introduction to standard indicators.

Explanation of Experiment: 

This demonstration shows that a mixture of indicators can produce a color change different from that obtained with the individual indicators, resulting in the colors of the rainbow appearing and disappearing.  The three indicators used are all colorless in acidic solution, but each produces a different color in basic solution.  Because these colors are close to the three primary colors, virtually any color of the rainbow can be produced by some combination of these indicators in basic solution.  The indicator solutions used are very concentrated, more concentrated than standard indicator solutions, producing intense solution colors.

At the start, each beaker contains one of the six rainbow color indicator solutions along with the acid/alcohol mixture.  Because the contents of the beakers are acidic, the indicators are colorless.  The initial addition of sodium hydroxide solution is not enough to neutralize the acid present, and the indicators remain colorless.  With the second addition of NaOH solution, an excess of base results, the solutions become alkaline, and the indicators become colored.  Then several drops of viscous acid solution are added to each beaker.  The dense droplets sink and are then dispersed by stirring, neutralizing the base and converting the indicator to its colorless form.  Adding more base to each beaker causes the colors to reappear.  Then the contents of the beakers are poured into the large glass container where a few drops of viscous acid solution have been previously placed, causing the resulting solution to become acidic and converting the indicator back to its colorless form.

Materials Preparation: 

Materials Required:

  • light background such as white table cover
  • towel
  • 600 mL Berzelius, tall form beakers (6)
  • 30 mL beaker
  • Large glass container
  • 12" stirring rod
  • 2 L pitchers (2 identical, non-transparent)
  • dropping bottles (9)
  • Kermit the Frog (optional)

Reagents:

  • phenolphthalein
  • thymolphthalein
  • p-nitrophenol
  • sodium hydroxide
  • concentrated sulfuric acid
  • glycerin
  • 95 % ethanol

Indicators:

  • RED: 1.5 g phenolphthalein + 3.0 g p-nitrophenol
  • ORANGE: 0.45 g phenolphthalein + 6.0 g p-nitrophenol
  • YELLOW: 6.0 g p-nitrophenol
  • GREEN: 0.6 g thymolphthalein + 6.0 g p-nitrophenol
  • BLUE: 1.5 g thymolphthalein
  • VIOLET: 0.9 g phenolphthalein + 0.4 g thymolphthalein

Mix the amounts listed above with 30 mL 95 % ethanol and store in dropping bottles labeled with the color indicated.

Acid-alcohol solution (0.05 M H2SO4 mixed with an equal volume of 95% ethanol):

  • Dilute 2.8 mL of concentrated H2SO4 to 1 L with deionized water. Mix with an equal volume (1 L) of 95% ethanol.

H2SO4-glycerin solution:

  • Dilute 10 mL of concentrated H2SO4 in 20 mL of glycerin, and transfer to a dropping bottle

Base solution (0.012 M NaOH):

  • Mix 1.00 g of NaOH with distilled water, dilute to 2.0 L, and transfer to a plastic pitcher.

Pre-demonstration Preparation:

  1. Label the beakers with their color, and place five drops of indicator solution of that color in the center of each beaker. Set them on the light background in rainbow order (ROYGBV) from left to right.
  2. Place a premeasured amount of acid/alcohol mixture (ca. 40 mL) in a small plastic bottle and place these in back of each of the beakers.
  3. Label one pitcher as base, and fill completely with base solution. Set it on the floor behind the lab bench out of view of the audience.
  4. Add 25 drops of H2SO4- glycerin solution in the center of the large glass container and set it to the left of the beakers.
  5. Set the stirring rod, towel, and dropper bottle of H2SO4-glycerin behind the arrangement of beakers and to the left.
  6. Set Kermit on the front of the table to one side so that the audience view will not be obstructed.
  7. If the indicator solutions dry on the bottom of the beakers, they will appear cloudy. Add a few drops of 95 % ethanol if this happens to keep them "invisible" on the bottom.
Presentation: 

Banjo strumming

Clean beakers with towel. There is enough time to do three of them.

Why are there so many songs about rainbows,
And what’s on the other side?
Rainbows are visions, but only illusions,
And rainbows have nothing to hide.

Pour acid alcohol mixtures into the beakers (ca. 40 mL). Be done by the end of the verse.

So we’ve been told, and some choose to believe it,
I know they’re wrong; wait and see.

Observe that there is no rainbow. Pick up the base pitcher.

Someday we’ll find it, the rainbow connection;
The lovers, the dreamers, and me.

After looking at the (base) pitcher in wonder, pour about 70 mL of solution into each glass. This is not enough to develop the colors.

Who said that ev’ry wish would be heard and answered
When wished on the morning star?
Somebody thought of that, and someone believed it;
Look what it’s done so far.
What’s so amazing that keeps us star-gazing
And what do we think we might see?
Someday we’ll find it, the rainbow connection,
The lovers, the dreamers, and me.

Starting with red, pour in sequence the colors of the rainbow.  Into each beaker pour in enough base to develop the color (approximately 150 mL), and display the colored solutions to the audience. Start with the word "Who" and finish with "me." When finished take a bow!

All of us under its spell;
We know that it’s probably magic.

Present the dropping bottle of H2SO4-glycerin to the audience.

Have you been half asleep and have you heard voices?
I’ve heard them calling my name.

Add two drops of H2SO4- glycerin to each beaker. Finish by "my name."

Is this the sweet sound that calls the young sailors?
The voice might be one and the same.
I’ve heard it too many times to ignore it.

Present the stirrer and stir out the colors of the rainbow in sequence. Finish by "ignore it." Present the colorless beakers and take a bow.

It’s something that I’m poised to be.
Someday we’ll find it, the rainbow connection;
The lovers, the dreamers, and me.

Pick up the base pitcher and quickly redevelop the colors in all of the beakers, again in rainbow sequence.  You will add about 100 mL of base to each beaker.

La-la-la......

As the song ends, remove the center two beakers (yellow and green) from the array.  Replace them with the large glass container. Pour the contents of the beakers into the large glass container, two at a time, pouring the last two (red and violet) as the song fades.

Hazards: 

The acid alcohol solution is flammable. Keep the container closed. Avoid flames or sparks. The acid (glycerin and acid alcohol) and base solutions can cause burns. Wear safety glasses. If you spill these solutions on yourself, wash it off immediately.

Disposal: 

The final solution may be disposed of down the drain.

Video: 

General Concept:

Type of Reaction:

Primary Reference: 
Creighton University Rainbow Connection Chemistry video.
Secondary Reference(s): 
Shakhashiri, B.Z. 1983, Chemical Demonstrations – A Handbook for Teachers of Chemistry, vol. 3 pp. 41-46.