Make and experiment with cabbage indicator

Discover the chemistry in your own kitchen, as you test for acids and alkalis using your own indicator. 

intro

What's it all about?

Lots of chemistry happens at the Crick, and knowing if a chemical is an acid or an alkali can be really important. Scientists use chemicals called ‘indicators’ that react with other chemicals to show, or indicate, if they are an acid or an alkali, and how strong they are, often by changing colour. One of these indicator chemicals can be found in red cabbage, and you can use it to test the things in your kitchen and see if they are acids or alkalis.

What you'll need

  • A red cabbage

  • A knife and chopping board

  • Two bowls

  • A kettle

  • A sieve

  • Things to test - any kitchen ingredients will work, but see below for some good examples!

  • What to do

    The simplest way to do this is if you’re boiling some red cabbage for dinner, just don’t throw the water away, that is your indicator. If you’re making from scratch though, we've got instructions for you!

    Things to test

    • Vinegar 
    • Fizzy drinks 
    • Tea 
    • Fruit juice (especially lemons) 
    • Tap water 
    • Eggs 
    • Bicarbonate of soda 
    • Soap 

    Safety tips 

    • Get an adult to chop the cabbage for you and handle the boiling water. 
    • It can get a bit messy as the indicators can stain so watch out for that too. Wearing an apron or old clothes might help! 

    What to do

    Quick steps...

  • Fill the kettle with water and get it to boil. 

  • Chop the cabbage up into small pieces, the smaller the better. You can tear it up too so you don’t have to use a knife.

  • Put all the cabbage in one of the bowls and pour the boiling water over it all.

  • Give the water and cabbage a stir every now and then, but leave it to soak until the water is cool.

  • Put the sieve over the second bowl and pour the cabbage and water through the sieve to remove all the cabbage. The cabbage can still be eaten, so add it to your dinner.

  • Your water should be a very deep purple. Dilute it with water, a little at a time, until you can see through it, but the colour is still clear. This is your indicator. 

  • Now you can test some ingredients to see if they are acids or alkalis. Put a little indicator in a glass and then add whatever you are testing. It will get redder for an acid, stay purple for a neutral liquid, and for an alkali it will turn blue, green, or even yellow for a really strong one.

    On this scale, acids would be on the left, neutral liquids in the middle, and strong alkalis on the right hand side.

    A colour gradient going from red on the left hand side, to purple, blue, green and eventually yellow on the right hand side.
  • The science

    Acids and alkalis are measured on something called the pH scale. It goes from 0 to 14. Things that measure from 0 to 6 are acids, things that measure form 8 to 14 are alkalis and anything that is 7 is neutral. 

    There are lots of different indicators, but this cabbage indicator contains a chemical called anthocyanin. The shape of the anthocyanin molecules can be changed by acids and alkalis, and different shapes appear as different colours. The different colours give us a visual sign of the pH of the chemical being tested. 

    Strictly speaking the pH scale, and your indicator, is actually testing if something is an acid or a base. Alkalis are just bases that can be dissolved in water, and the bases found in a kitchen are typically alkalis.