An acid-base rainbow wand is an easy and colorful chemistry demonstration which illustrates the range of colors available for a pH indicator solution.
Liquid Rainbow Wand
Using a liquid indicator yields a quick rainbow, but the result is not very long-lasting because the pH stabilizes.
Take a long plastic or glass tube and fill it with universal indicator solution. Add a few drops of 0.02M HCl to one end of the tube and seal it with a stopper. Add a couple of drops of 0.02M NaOH to the other end of the tube and seal it. The universal indicator responds to the pH gradient by forming a lovely rainbow.
You can invert the tube a few times to speed things up.
Easy Rainbow Wand Home Chemistry Project
You can get a similar result using home chemistry. Fill a clear straw with red cabbage juice. Add a little lemon juice or vinegar to one end of the straw (acidic ingredient). Add a few drops of baking soda or laundry detergent solution to the other end of the straw (basic ingredient).
Simple Gel Variation for a Rainbow Test Tube
A variation on the project is mixing in a pH indicator with gelatin or agar. After the gel sets, apply chemicals with different pH values to opposite sides of the mixture (use a pipette or syringe to deliver solution to the bottom of a test tube). Alternatively, set up the demonstration using sodium polyacrylate gel beads (water beads) with indicator solution. Either the gel or the beads slows the progression of the pH change, so you don’t need to use a super-long tube to get a lasting rainbow effect.
References
- Foster, L. S.; Gruntfest, I. J. (1937). “Demonstration experiments using universal indicators”. Journal of Chemical Education. 14 (6): 274. doi:10.1021/ed014p274
- Zumdahl, Steven S. (2009). Chemical Principles (6th ed.). New York: Houghton Mifflin Company.