Procedure to produce 7 colours of UI in a single test tube:

1. Pour some dilute sodium carbonate and green UI into a test tube which will turn purple.
2. By using a long plastic dropper, add slowly ethanoic acid (4% like those sold commercially by colourless) into the test tube  by dripping   it in along the side of test tube.
3. Add more green UI, if necessary.
4. Continue to pipe in ethanoic acid until the top portion of the solution turns red . Add more green UI, if necessary. 
5. Voilà! Almost all the colours of the rainbow in a test-tube. One of our students commented that we have captured the rainbow in a test tube.

There are two ways in which these 7 test tubes of the seven colours of UI can be made. Do you know how this can be brought about? 

The 2 methods are quite different from the procedure listed above which can only bring about the multi-coloured test tube.

Seven colours of Universal Indicator (UI) all in one test tube. Universal indicator is a solution that can test for the alkalinity or acidity of liquid using the pH scale. Red represents most acidic at pH 1. Blue - most alkaline at pH 14. UI is green at neutral pH 7.

In the single test tube on the left, seven colours of UI can be seen. This shows that the liquid in the test tube is alkaline at the base of the test tube and acidic at the top.

Question: Why does green UI turn purple (violet) in a solution of sodium carbonate, the formula of which is Na₂CO₃?

An original experiment from PL Education Centre & PL Enrichment LLP.

Lishan shows off her colourful test tube of universal indictor