Kitchen chemistry

This activity will take 30 minutes, is for ages 7 to 10 with supervision needed. You can do this activity at home.

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 something acidic such as vinegar and something alkaline such as dishwashing liquid work well.

What to do

Ask an adult to fill the kettle with water and get it to boil.
Ask an adult to 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 ask an adult to pour boiling water over it all.
When the water has cooled a little, give the water and cabbage a stir, 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 you could 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 kitchen chemicals. Put a little purple indicator in a glass and then add whatever you are testing. You could try lemon juice, vinegar or dishwashing liquid. Your indicator will get redder if something's an acid, stay purple for a neutral liquid or turn blue, green and even yellow for an alkali.

What's happening?

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.

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.