If you boil a red cabbage (with plain water), you get a beautiful purple coloured liquid out of it.

This liquid is sensitive to pH. You can use it to measure the acidity or alkalinity of substances - it shows good colour range over a wide pH.

Since I boiled red cabbage for dinner, I used the liquid to demonstrate this:

1. Leftmost : ammonium persulphate - highly acidic, turns the liquid a dark red
2. 2nd from the left : apple cider vinegar - acidic, turns the liquid a pinkish colour
3. middle : the red cabbage liquid - a beautiful purple
4. 2nd from the right : sodium bicarbonate - slightly alkaline, turns the liquid a wonderful blue
5. Rightmost : ammonia - very alkaline, turns the liquid a greenish colour

I was surprised at how alkaline the ammonia turned out to be - I knew lye (sodium hydroxide) would turn it green, but I didn't realize ammonia would.

The only error I made was in preparing my solutions from right to left. The ammonia fumes slightly affected the blue and purple mixtures (making them a little greener than they should be - if you look carefully, you can see a little green tint around the border of the liquid in the middle spoon. This happened because I left everything sitting there as I went to the basement to look for the ammonium persulphate and the ammonia fumes diffused into the other solutions).

To preserve the indicator, mix it with an equal part rubbing alcohol and store it in the fridge (much to my wife's chagrin).