======pH Rainbow Tube======


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**Materials: **{{$demo.materials_description}}\\
**Difficulty: **{{$demo.difficulty_description}}\\
**Safety: **{{$demo.safety_description}}\\
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**Alternative titles:** Universal Indicator Color Column 
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====Summary====                   
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====Procedure====  
  - Stopper one end of a long demonstration tube securely and place it upright on the table.  
  - Fill the tube almost to the top with diluted universal indicator solution (green).  
  - Add two drops of dilute hydrochloric acid (0.1 M) and stopper the tube.  
  - Invert the tube and observe the development of acidic colors (red–orange–yellow).  
  - Remove the stopper at the top, add two drops of dilute sodium hydroxide (0.1 M), restopper, and invert again.  
  - Observe the full spectrum of colors from red through green to purple as the solutions mix.



====Links==== 
pH Rainbow - SSERC Chemistry: 
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pH Rainbow Tube - FlinnScientific: 
{{youtube>ZT9le3AUI7E?}}\\

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


====Variations====
  * 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.



====Safety Precautions==== 
  * 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.




====Questions to Consider====
  * 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.)