What’s called Red but really Purple and comes out Blue?

by Deb on May 6, 2011

Red Cabbage

Red cabbage of course.

Red cabbage is beloved of science teachers the world over because it is incredibly simple to boil it up and make red cabbage indicator to test acids and bases. The beauty is not just the bright colours, but it actually reacts differently to each. Many natural indicators such as tea will change for one pH level, but red cabbage covers all the rainbow.

The chemicals that react to the acids and bases are generally reds, blues and purples so things like beetroot, blueberries and mulberries also work. Even rose petals and turmeric, although that’s bright yellow! The only downside is that I now have a pot of overboiled cabbage to get rid of, and I have a tendency to forget and burn the saucepans.

It’s so simple, you just chop it and boil it up,


and you have a big pot of purple blue indicator.


Then you can start testing all sorts of things from around the house. Here we have vinegar, orange juice, soda water, cleaners, soap, laundry powder, sodium peroxide and probably others. It was quite funny, the little girl wasn’t all that interested – she’s played with coloured water lots of times –  until I got the first one to turn pink!


Our rainbow of results. Including the foaming one where I added vinegar to the laundry detergent to see if I could get it back to red. Now the rest of the indicator is sitting in yet another cool drink bottle (that my husband has asked me to clearly label so he doesn’t accidentally drink it) waiting for the weekend, when we can find even more chemicals to test and mix together.rainbow

You should try it – you don’t need anything special just some little containers, bottles from around the house, mix them together and off you go. Do you think food colouring would act as an indicator? Or what else from the pantry?

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{ 4 comments… read them below or add one }

jasmine May 7, 2011 at 5:18 am

Hey! Don’t suppose you can utilize this method to test the ph of a fish tank? lol, i grow tired of passing by the overpriced ph test kits at walmart =/
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Deb May 8, 2011 at 11:18 am

Depends how exact it needs to be. You can work out a pH scale comparing it to another scale but I don’t know how fine it is. If your fishtank needs to be within a few points it’s probably not a good idea!


Hear Mum Roar May 7, 2011 at 12:36 pm

This looks brilliant! My kids would love it


Louise at Tales from the Sandpit May 9, 2011 at 8:06 am

Hi Deb, thanks for this clear explanation about how to do it – but how do you explain it (because they will ask WHY) – I understand that the cabbage water reacts to the chemicals according to what sort of chemical it is – acid or alkali – what’s that to 5 year olds?
And do you have any nifty ideas for seeds – apart from the obvious growing, grinding, cooking, tasting things.


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