Try this experiment and see how fast the colours change!
To make indicator ice blocks:
1 Slice half a purple cabbage and put it in a saucepan with 2-3 cups of water.
2 Bring it to the boil and then turn off the heat. Leave it to cool.
3 Pour the cabbage water through a strainer into a jug.
4 Pour the liquid into an ice cube tray and freeze.
For the experiment:
5 Take three glasses: Half fill one glass with water. Fill another with half water and half white vinegar.
6 In the third glass make an alkaline solution: add 1 teaspoon of baking soda to half a glass of water and stir to dissolve.
7 Drop one of your indicator ice blocks into each glass and watch what happens to the colours.