Saturday, 21 February 2009

Colour changing magic

OK, so really it's chemistry, but the littlies prefer to think of it as magic.

At this time of year, most weekends we have a big roast and frequently red cabbage is on the menu; usually I use any leftover cooking water to add into stock, but last time we decided to save it up and freeze it into icecubes for a little experiment - red cabbage indicator solution!

So, we set up 3 glasses with warm water in them; one we left as is, one had a teaspoonful of bicarb (baking soda?) dissolved in it, and one had a teaspoonful of citric acid - though lemon juice would work just as well.

We added a cabbage juice cube to each glass and watched them thaw ...

We determined that acidic solutions turned red, alkaline ones turned blue, and neutral ones were somewhere in between, and they were all very pretty.

If you wanted to do this properly you ought to cook the cabbage without vinegar (which is acidic) as that does alter the pH of the indicator solution a bit - so the neutral water probably wasn't entirely neutral - but you can get bogged down in detail with tiny people I think, and all they really needed to see was 3 different colours.

We did some neutralisation experiments - what happens if we mix the acidic and alkaline solutions together? What if we add some baking soda to each glass? Or some more citric acid? I will confess, the washing up bowl was employed for some large scale chemistry.

For those wanting to know the science, red cabbage (and the juice!) contain an anthocyanin whose structure changes depending on the pH, causing a colour change. It's the same kind of molecule that makes hydrangeas pink or blue depending on the pH of the soil. You can get more details here.

Or you can just enjoy making pretty colours!


  1. Truly brilliant Val!!!! You are inspiring (and really rather clever too!!!!)

    Lucy xxx

  2. That's fantastic. Those two little cookies are going to be *so* bored by the science lessons at school because they get much better (and more fun) lessons at home! Have you considered being a science teacher? x

  3. Oh Val - you've got me very excited. Nerdy urges are definitely answered for the day. Hurray! Hmm though, have you ever come across dishy nerd Brian Cox? Swoon!

  4. Fabulous. All those lovely colours and it's educational too!

  5. That's amazing and SO much cooler than the litmus paper test *yawns at the memory*

    I didn't realise hydrangers (sp.?) were different based on the soil, although it might explain why the ones in my parents garden were pink in parts and purple in others. Wish I'd asked my mum this whilst I was still at home....


  6. Woohoo - fantastic geekery in the Dottycookie kitchen!!!

    I agree with the little Cookies - the colours are very pretty. Thanks for the nerdy link :-)


  7. oo! Loving a bit of pretty coloured nerdism.

    Hmmmmmm, CK may just agree* to cabbage being cooked in the house in the name of science.

    *No cabbage cooked in the house was just one of our unstated wedding vows.

  8. I wonder what the little cookies will be when they grow up?

  9. gosh how clever you are. I will have to hide behind the sofa when you come round.

  10. How fun! You're so very clever and I love that the littles prefer to think of it as magic!!!

  11. Cabbage indicator - I NEVER knew that!

    I must say that I realise now why my two have ended up on the Arts side as it never occurred to me to do Science lessons in the kitchen when they were little. Two potential Astrophysicists lost forever.

  12. I want to come and live in your house. My cabbage water goes in the gravy, your way is much more fun.

  13. Gosh! you make science so much fun!

    Have just heard that mega-clever Ms Trimble whose team in in the University Challenge final tonight, interviewed on Woman's Hour - she got one question right because she remembers chemistry experiments she did with her mum ;-)

    I love the colours

    PS I see Silverpebble's still getting excited about Prof Cox :-)

  14. very very pretty... lovely heart glasses!

    (that is my interlectual comment for the day!)

  15. Can't wait to try it!! I've now officially purchased the red cabbage for this very purpose!!! (Thank you, Ms. DottyV!)

  16. I love all your experiments Val and i shall be making a note of this one ready for the next school break when we need things to do. It's great that something simple can produce hours of fun!

  17. I think I'll just enjoy making the pretty colours bit! This takes me right back to my days at school, I always used to get confused which went red and which went blue ... If you have any easy peasy tricks for passing a very imminent GCSE physics module for my eldest daughter, please tell!!!
    Sorry i missed you last week, half term is never long enough. Hope you had a great cup of tea in my honour and a very big slice of cake!

  18. Love it. Another great one to file away for a rainy day experiment!

    By the way, am very impressed your family have a roast every Sunday. We average one every 6 months in this house...

  19. What a great experiment. You should be on that new programme that starts this week about transforming the way science is taught in schools - you'd be great!
    Cathy X

  20. I think you have just won a science fair prize ;-)

  21. Sorry I missed your post Dotty but i'm glad I found it, as your experiment sounds very good and I think Em might like to try it out during the holidays.Pretty glasses too. Jane x

  22. Answer for Ragged Roses:
    acid = red (AR)
    Alkaline is base, and base = blue (BB).

    Question for DottyCookie: Would pouring acidic water on the hydrangeas change their colour? Or vice versa, with alkaline water?

    janeyknitting AT yahoo DOT ca


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