In lesson 13 you met indicators: dyes that change colour with pH. Red cabbage juice is one. It is purple, and it changes colour when you add an acid or a base.
An acid gives away hydrogen ions, H⁺, and a base picks them up. The pH scale runs from 0 to 14: 7 is neutral, lower numbers are acidic and higher numbers are basic.
Why the colour changes
The anthocyanins in red cabbage carry -OH groups on their rings. In strong acid, an anthocyanin keeps all its hydrogens and carries the positive charge you saw on cyanidin. This form absorbs green light, so the juice looks red.
As the pH rises, bases take H⁺ away from the -OH groups one at a time. Each H⁺ that leaves changes which bonds in the run are double and which are single, so the molecule absorbs a different colour. Near neutral the juice looks purple, and in a weak base it looks blue. In strong base the middle ring breaks open, and the new forms are yellow. On the way from blue to yellow, around pH 10, the juice looks green.
| Added to the juice | pH, about | Colour |
|---|---|---|
| lemon juice | 2 | red |
| vinegar | 2.5 | red-pink |
| nothing | 7 | purple |
| baking soda | 8 to 9 | blue-green |
| washing soda | 11 | green |
Quick check
That's the end of lesson 15
In this lesson you found out:
- How a coloured molecule absorbs some colours of light and sends back the rest
- Why long runs of alternating double bonds give carrots and tomatoes their colour
- Which molecules colour leaves, berries and red cabbage
- How red cabbage juice changes colour with pH