Rainbow tube

The liquid in the U-tube is painted gradually in different colors from different ends of the tube

Scientific name: Universal indicator in U-tube changes color, when hydrochloric acid and sodium hydroxide are put in different sides of tube


Universal indicator U-tube

by ChemToddler

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Wear eye protection throughout. Prevent skin contact with acid and base solutions.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

H2Ind → HInd- → Ind2-

H2Ind ← HInd- ← Ind2-

Step-by-step instruction

  1. Take a long tube, hold it vertically, and place one plug at the bottom end of the tube.
  2. Partially fill the tube with the indicator solution.
  3. Add a few drops of acid to the end of the tube and close it by another plug.
  4. Turn the tube over. The air bubble in the tube mixes the solution.
  5. Remote the plug (which now is on top) and add a few drops of base to the end of the tube.
  6. Close the upper end of the tube and turn the tube over again.
  7. The rainbow colored solution in the tube is formed.

Scientific background

There is a gradient of acid and base concentrations in the long tube. Because of the reaction between base and acid different pH-zones are formed. The indicator changes color in these zones.


Published on 23 December 2014

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst