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pH Rainbow Tube

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Extracting an indicator from red cabbage
How to create a beautiful rainbow of colors in a demonstration tube using universal indicator and a dilute acid and base?

Prepare a dilute solution of universal indicator in distilled or deionized water. The solution should be green in color and dilute enough to see through.

1. Stopper one end of the demonstration tube with a rubber stopper. Make sure the tube is securely sealed and clamp the tube vertically to a support stand. Place an empty beaker under the tube to protect against leaks.
2. Fill the tube to within 3–4 cm of the top with the diluted (green) solution of universal indicator.
3. Add two drops of 0.1 M hydrochloric acid solution to the tube, stopper securely. Remove the clamp, invert the tube, and observe the colors produced.
4. Remove the top stopper (which used to be on the bottom) and add two drops of 0.1 M sodium hydroxide solution to the tube. Re-stopper securely, and invert the tube.
5. Clamp the tube to the support stand again and observe the full spectrum of indicator colors spread throughout the tube.

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