This demonstration produces a rainbow of colors inside a long glass tube by adding acid and base to a universal indicator solution. The color gradient shows how pH changes from acidic (red) to neutral (green) to basic (purple).

1. Stopper one end of a long demonstration tube securely and place it upright on the table.
2. Fill the tube almost to the top with diluted universal indicator solution (green).
3. Add two drops of dilute hydrochloric acid (0.1 M) and stopper the tube.
4. Invert the tube and observe the development of acidic colors (red–orange–yellow).
5. Remove the stopper at the top, add two drops of dilute sodium hydroxide (0.1 M), restopper, and invert again.
6. Observe the full spectrum of colors from red through green to purple as the solutions mix.

pH Rainbow - SSERC Chemistry:

pH Rainbow Tube - FlinnScientific:

📄 pH Rainbow Tube - FlinnScientific: https://www.flinnsci.com/api/library/Download/404d04843ea1428186cdd4dcabee3d97?srsltid=AfmBOooCRWacOEQog3wo-ZmOajo6rwbUm5Bicu3hr3OFtN4jYWeqhZ9D

• Use weak acid (vinegar) and weak base (ammonia or sodium carbonate solution) to show buffer effects.
• Let the tube stand vertically without mixing and observe color changes over time.
• Try repeating the inversion multiple times and ask students to predict how many inversions are needed to mix the colors evenly.

• Dilute hydrochloric acid and sodium hydroxide can irritate skin and eyes. Avoid spills and wash immediately with water if contact occurs.
• Always keep the bottom stopper firmly pressed against the table before inserting the top stopper to prevent spills or pressure accidents.

• Why does adding acid shift the colors toward red and adding base shift them toward purple? (Universal indicator changes color depending on hydrogen ion concentration.)
• How does this demonstration illustrate neutralization reactions? (Acid and base additions balance out to return the solution toward green at pH 7.)
• Why is universal indicator better than a single pH indicator for this demonstration? (It contains a mixture of indicators that cover the whole pH range.)