Did you know that the humble red cabbage can be used to make an indicator solution which can test whether chemicals are acids or alkalis? Make your own and test it with a class!

You can make the indicator in advance at home and bring it in. Or the children could have a go at making it themselves.

You will need

• A red cabbage
• Saucepan
• Sieve
• Filter paper
• plastic containers
• pipettes
• selection of household chemicals to test.

What to do

1. Cut the red cabbage up into rough chunks.

2. Put the cabbage into a saucepan and cover with water. Simmer for about 15-20 minutes.

3. Strain the cabbage into a large jug. Leave it to cool.

Safer Method

If the children having access to hobs and saucepans is an issue you could boil water from a kettle (or boil in advance and keep it in a flask). The children chop some cabbage and put it in a bowl. You can pour the hot/boiling water onto the cabbage and let it stew for 20 minutes as the water cools down. Then the children can sieve it and use the liquid in their tests. It still works well enough.

To use the indicator as a liquid

Simple pour small amounts of the cabbage indicator into small pots (Tip: an ice cube tray works great for this!).

Use a pipette to add small amounts of household chemicals such as vinegar, lemon juice, bicarbonate of soda (dissolved) etc.

In an acid, the solution will turn red. In an alkali it will turn green.

To make indicator paper

Pour the liquid into a wide bowl.

Use filter paper (white coffee filters will be fine, not the brown ones!)

Soak the paper in the liquid for 5-10 seconds. Lift it out carefully and allow the excess to drain off.

Put the filter papers to one side and let them dry out.

Cut or tear the paper into strips (or just pieces!). These can be dipped into solutions to test, or solutions can be dropped onto them using a pipette.

Acids will turn the paper red. Alkalis will turn it green.

How to use this with a class.

Test out small amounts of chemicals found around the house such as cola, vinegar, lemon juice, hand soap, cleaning spray etc. The children can test to see which are acids, which are alkalis and which are neither.

Take care. Gloves should be worn if handling chemicals. Don’t use bleach, it won’t work!

How does it work? What’s the science?

Why does cabbage work as a pH indicator? Red cabbage contains a pigment known as anthocyanin. Anthocyanin changes colour when it comes in contact with an acid or a alkali (base). The red colour mean that the pH is acidic. Green means it is an alkali.